When it comes to ffp and hunting close up, in low light, I think the reticle plays a huge role. You can’t expect to see a fine reticle at low power, and low light, on a ffp scope. If how you shoot things is likely close, fast, in low light, best bet to stick to a sfp scope.
I also agree with you in that a lower power SFP scope isn’t as detrimental as a high power SFP scope. On a max 10x scope, if I’m taking a shot that requires any accurate holding, I’m probably going to be on max power anyways. Animals have a fairly large x ring, you don’t have to thread the needle on those close shots to make meat.
On the other side I had a sfp scope that went to 18x and hated it. It had to be on 18x to have accurate holds. That made it more difficult.
Then I have my swfa 3-9. Ffp, low power. Thick reticle though that is usable for me in many situations. Not the best for being precise long, by the thicker reticle, combined with the larger posts of the reticle, make closer shots at low power doable. On top of that I can leave it on 6x and do all I need over both ends, and be happy.
Going forward I’m going with your train of thought. 3-15 is probably on that ragged edge. I’d let your hunting style dictate. If your hunting more open areas, where you likely have time to setup shots I’d think ffp might be an advantage. Closer in or in timber, sfp is what I’d be looking for. Personally if I’m getting a 15x or higher max magnification scope I’m probably going to want ffp, just based on how I hunt.
Illumination makes a difference, but I don’t count on it to make a shot if I don’t have to.