Ah... this topic...
There is no perfect spot to carry while riding. Carry position is something you have to decide for yourself according to your own situation. There will be positives and negatives no matter what you decide. You are adding additional levels of risk when you carry while riding, so you have to mitigate that risk just as you already do for the act of riding itself.
If you want to carry on your person, the main downside is that you have to decide what part of your body you are willing to likely have considerable extra damage, due to having a big chunk of steel between you and whatever you impact in an accident. There is no way around this. Also, you will feel lopsided until you grow accustomed to carrying. The upside is you are typically quite unrestricted to move and go about your daily routines, both on and off the bike. The firearm is always with you and accessible so therefore you do not have to worry about it any further.
If you want to avoid the "increased injuries" scenario, then you will have to find a way to carry on the bike itself, which comes with it's own upsides and downsides. Obviously this is usually going to be a more comfortable and convenient solution, as you can carry in a backpack, tank bag, saddle bags, holster attached to the bike, etc. Lots of options. Unfortunately, most "off your person" solutions are going to have the problem of the firearm not being quite as readily available, which negates the whole purpose to an extent, and what happens to the firearm during an accident if it is physically on the bike in a holster or bag? You definitely do not want to lose the firearm for all the reasons I doubt I need to cover here. Also, transferring it back and forth between yourself and the bike at every stop is cumbersome at the least, and if you need to keep it concealed then the situation is a whole lot more difficult.
There is another important reality to consider whether you carry on your person or on the bike. Many people are unaware of this but in some places it is standard procedure/policy/whatever that when first responders, paramedics, etc., arrive at the scene of an accident, if a firearm is found they are not allowed to render aid until law enforcement arrives. I'm sure this is much more likely in some of our "less constitution friendly" states than others, but this means there is a possibility that if you are in an accident and your firearm is ejected from either your bike or yourself, you may very well be left on the pavement longer than you can afford before anyone will help you. I have yet to see or hear of this actually happening, but it is a possibility of which you need to be aware.
As for carrying positions, you will likely end up with more than one. Different situations call for different types of carry. Again, it all basically hinges on risk and what you are willing and able to accept/mitigate. Below are options I have either used or witnessed others using.
Probably one of the most comfortable methods while riding, regardless of riding position. Not as easily concealed as one might think. (Bulge will be noticeable with most motorcycle jackets.) Fairly quick access. Looking at some seriously broken ribs and/or arm if impacted in an accident. Not likely to be thrown from your person.
"Gun Pocket" Carry:
Again, a comfortable method for any riding position but if impacted in an accident could yield broken ribs and/or arm. Fairly quick access. Not likely to be thrown from your person unless the jacket/vest/etc is unzipped more than a few inches.
In or Out of Waistband (IWB/OWB) Carry:
Usually comfortable regardless of riding position, so long as positioned to left or right side. If the firearm is far forward, such as appendix carry, forward riding positions will likely not be comfortable. If the firearm is far back, in the small of the back or kidney areas, more reclined riding positions will likely not be comfortable. Hip and/or spine injuries likely if impacted in an accident. Fairly quick access. Holster quality and retention will affect likelihood of ejection in an accident.
Comfortable option that works quite well. Unfortunately unless wearing pants with a built in thigh holster, this is not concealable. You will draw attention, both good and bad. (In my opinion, any attention drawn is bad.) Fairly quick access. Possible broken leg if impacted in an accident. Unlikely to be ejected in an accident unless holster is of poor quality.
Less comfortable option due to location and typical vibrations from road surfaces. Wind will likely not allow full concealment. Will not work with most riding boots. Fairly quick access if not inside a boot. If impacted in an accident, broken leg, ankle, and/or foot is likely. Likely to be ejected in an accident unless holster is of high quality design.
You'll never know it's there aside from the weight. No one else will know it is there period. Fairly easy to keep on person, though will require personal awareness not to leave it unattended. EVER. Basically not accessible unless off bike. Extremely restricted access while on bike. Highly unlikely to cause any personal injury if impacted in an accident. Extreme circumstances could cause the firearm to come free of the backpack during an accident. (Example: Ejected through hole torn in pack during accident.)
Tank Bag Carry:
You won't know it's there unless you open the tank bag and look, and neither will anyone else. Fairly easy to keep on person, though will require personal awareness not to leave it unattended. EVER. Restricted access while on bike. Unlikely to cause personal injury if impacted in accident. If tank bag is secured by magnets, fair likelihood that bag will be ejected during accident, but firearm should remain inside.
Tail Bag / Saddle Bags Carry:
You won't know it's there unless you open the bag and look, and neither will anyone else. Fairly cumbersome to keep on person, IF they are easily detached and you have the willingness to do so. If the bags stay on the bike, you will have to transfer the firearm from them to your person and back at every stop, or take the risk of leaving them unattended. (Locks only keep honest people honest!) Obviously will require personal awareness not to leave detached bags accessible to others. EVER. Highly restricted access while on bike. Unlikely to cause personal injury if impacted in accident. Not likely to be ejected during an accident unless bags are not properly closed or secured.
I will not cover all the options here, just focusing on the general concept. An easily accessed position will mean it is also easily seen and accessed by others. A well hidden position will likely highly decrease accessibility to the firearm, to both yourself and others. Fully hidden will likely mean no access while on the bike. Only the best holster quality and design will have any chance of keeping a bike-mounted firearm from being ejected in an accident. If the firearm is ejected, it will likely cause significant damage to whatever it impacts, including yourself or others. Chances of loss of firearm in accident are high. Personal awareness to never leave the firearm on the bike while unattended is paramount. I would suggest that the firearm be removed and placed on person or other form of carry any time the bike is not being actively ridden.
Don't try to find an end-all way to carry while riding. Some situations may allow you to carry the way you normally would while not riding. In other situations it may be best to switch to another method. It's your call.
That about covers it. As stated in the beginning, these are my opinions based on my experiences. These are definitely not all the possibilities for carrying while riding, but they are the ones which I have either practiced or directly observed over time.