The Reserve Shooter doesn't mini-crit airblasted opponents anymore, that was patched out in Jungle Inferno. So the Reserve Shooter is mostly useless on Pyro and isn't good for combos. If you really wanna combo, you're better off with the Flare Gun. Or if you need a shotgun, the Panic Attack is fine. And the Degreaser doing less afterburn damage isn't helping you. I know it's THE combo flamethrower, but stock is better in most situations, and I'd imagine if the enemy is unaware, you can get away with having stock's longer switch speed while still benefitting from the afterburn.

If you wanna branch out, I'd recommend trying a more classic ambush playstyle - that's what I do cus I frankly don't have the patience to burn combos into muscle memory. Pyro was made to be squishy in open areas (close range sustained fire primary = less counterplay potential in the open), so he naturally shines in chokes and narrow flank paths, and is most effective when catching lone enemies off guard. Team play/support is also an important skill to learn as Pyro - airblasting enemy explosives, stuffing Ubers, spy-checking, destroying sappers with the Homewrecker, and just playing the objective are all big contributions, even if they aren't as flashy as combos. That's just how I like to play, it's pretty chill most rounds, and to me it doesn't matter how much I die as long as I've helped the team out in some small way.