For sheer aftermarket parts you can't beat the Glock line.
I agree the Sig P320 line is starting to take off in the aftermarket, and a lot of offerings are possible.

Here are some off the wall suggestions:
Get polymer 80 kit and put in whatever trigger/spring mechanisms you want https://www.polymer80.com/pistol-fra...ild-shoot-kits
Faxon Firearms makes some Gucci Glocks https://faxonfirearms.com/fx-19-pistols/
Consider the Warrior Poet branded handgun https://www.sportsmansguide.com/event?key=warrior-poet
Get the SIG X5 Legion - I have one, it's great. https://www.sigsauer.com/store/p320-xfive-legion.html
Get a P320 and modify it - (apex trigger bar, Grey Guns trigger, etc) - I personally like the feel of the Wilson Combat P320 grip more than the others.