So, stiff arrows are ok and a tight group anywhere on the target is the goal?
That make sense since it is the goal I was given by a coach many years ago when I first attempted to learn archery.
Let me go into a bit more detail. I was on my phone when I first responded and I am not very good with the small keyboard.
I make my form changes with light limbs, usually 18 pounds that give me about 24.5 at my 32 inch draw length. I would go lower but that is difficult to do with my long draw length. I follow the approach taught to me by Sandy McCain:
Work on form at low weight
Move up 1/2 pound at a time (This occurs over a long time period. It can take months to get to full weight. Half pound is the change that the body does not notice.)
Go back if form changes after a move up in weight
Stop when you can't go higher without breaking form
I spend a lot of time shooting light bows. One thing I don't worry about is tuning them. I am not competing, though I have competed with a light bow in the past when I was mid process. The goal is to work form, not to shoot 10's. Also, it is safe to shoot a stiff arrow. Weak is where is it becomes a hazard. You can put a stiff arrow in the middle of the target. It just takes better execution, as such, it is a good indicator of improvement. Mistakes are amplified with stiff arrows so you get really good feedback. I often start a training session with a fairly large group but tighten it up as I go. That is the result of improvement made during practice. The key for form practice is whether the shot felt right, and to a slightly lesser extent, where did it end up. If I was shooting a tuned light weight bow at short distance, I may not be able to see a difference in group. Shooting stiff arrows increases the variance.
The message is if you are practicing form then focus on that, and not tuning a practice bow (other than basic setup). Shoot the arrows you have. That reduces the expense of buying some 1000 or higher spine arrows for 16 pound limbs (if you can find that spine long enough for your draw length). This way you also don't have to reset your bow up when you go back to your regular limbs. High spine limbs will require a rest and center shot change since they will be very thin. Also, if you sight with your arrow, your site picture will stay the same. I really don't see any upside to trying to tune a bow to my form limbs, and have never done so.
The only downside is that you may wear out your fletching, but reflecting is much cheaper than new arrows. I would not worry about reflecting any over worn arrows until you get back up to weight. Also, I'll shoot anything, vanes, feathers, spin wings, it does not make a difference when doing light weight form practice.
This is my opinion from using this method of learning over many years. Others may have different opinions. It has worked for me shooting 430 spine arrows from 18 pound limbs with a 24.5 pound holding weight.