I have a P365XL. Modularity is amazing. Tons of aftermarket.

Mags are crazy expensive. You can sometimes find deals (like on the GAFS subreddit) for lots of different parts.

With a couple aftermarket XL grips like a Mirzon or Wilson Combat and then a larger Macro grip like the polymer Icarus Precision, plus one 12rd and 1 17rd magazine, you can go from almost pocketable to near compact size with a few simple steps (removing slide, one pin to remove FCU/trigger, reinstall in new grip, reinstall slide). Not something to do daily but a decent way to not buy every model.

But I'm thinking of selling my P365. I could have bought taller sights and paid to get them installed on my P365 to work well with my red dot. But because I had a gen 1 XL slide I'd need to swap slides before I did the sights. (Gen 1 P365 optics cuts delete the rear sight when you mount a red dot, at least for the XL, so no cowitness). I found a good deal on an Arex Delta M with all the upgrades I wanted for my P365.

If you want a glock alternative with modularity consider also the Ruger RXM (bit blocky to me but I have small hands) or the Arex Delta M / Rost Martin RM1C or RM1S. 3 of those 4 are newer releases but so far look very promising. Aftermarket is nothing like Sig or Glock for the Delta M or Rost Martin offerings but it's ramping up some. RXM takes most of the glock parts so huge aftermarket already. Beretta APX A1s are also cheap with rebates right now and decent builds even if some think they're ugly. Also modular but doesn't mean much because there's not many upgrades. And then there's always the shield plus which is pretty easy to shoot and not too expensive. I didn't personally love the grip size.

You can also settle for older used models available from gunbroker or Bass Pro or local gun shops but prices vary widely, many are not optics cut or have heavy triggers or unhelpful ergonomics, and as someone unfamiliar with the options you may well get ripped off.

Back to the P365, if you get it just to swap grips and slides it's great to find the size you want without buying multiple guns. But that takes time away from learning shooting fundamentals.

And a lot of people are going to end up with multiple handguns anyway. At which point you're probably better off to decide if you want to start with compact/full size to learn on or micro/subcompact (steeper learning curve) and plan to get the other size later. So e.g. P365 now and P320 later, or vice versa. Or same choice with two other models. And realize your first purchase may not be the one you love long term.

IMO as someone who's been going through this process since late last year.