There is one sure way to eliminate flinching, not just with a flintlock, but with any firearm. It takes practice, and it takes discipline. If you can train yourself to do it, I guarantee you will never flinch. And you will probably shoot better as well.
When your sights are on target, apply pressure to the trigger. As the sights drift off - and they will -, hold that pressure but do not increase it. As the sights drift back where you want them again apply more pressure. Continue the procedure until, without knowing exactly when it is going to happen, the rifle fires. It should be a complete surprise when the gun fires, and the sights will be on target. It’ll all be over before you have a chance to flinch. A good way to practice is to make a few wood flints so you don’t use up those expensive real ones.
Forget that click, boom often associated with flintlocks. With a good lock (and it looks like you have one) there should be no noticeable difference in lock time between a flintlock and a percussion. My main go-to rifle for matches has a Chambers round face lock with a hammer throw that seems twice as long as locks on other rifles I’ve had. Even so, I never perceive any lag as long as I’ve done my part.
Doing my part is pretty simple. 1. Prime the pan. 2. The really, really important part, PICK THE TOUCHOLE to make sure it’s clear. I usually like to make sure the pick is well into the charge to make sure it didn’t get crunched too tight when the ball was rammed home.
It’s a matter of personal choice, but if that was my rifle I wouldn’t install sling swivels on it. They look okay on muskets and some Germanic rifles but they are really out of place on that style of rifle. There are guys out there who make slings that fasten on the rifle by way of loops on each end of the sling. I’d at least consider something along those lines.
As far as removing the pins is concerned, I would be really surprised if there is any taper to them. It shouldn’t make any difference which way you tape them out.
Your best bet for balls is going to be .440 or .445.
One thing I’d do early in the game is get a different tip for the ramrod. Track of the Wolf (and others) sell a tip that is just a button. The advantage is that you won’t have something sticking out past the muzzle to snag on brush.
For the length problem go the route of a lot of chunk shooters. Some build a box to stand on, and some dig a hole in the ground for the butt of the rifle to sit in. No kidding, they really do. Just kidding about you doing it though.