It’s hard to give a definitive recommendation as what one person finds concealable another person finds too big. That said I would say Glock 19.
There are a lot of options out there in the striker fired polymer framed pistol market. I’ve owned a Springfield Armory XD and XDm, SIG P320 and P365XL, Walther P99 and PPQ, FN FNS, HK VP9, S&W M&P 1.0 and 2.0, Beretta APX, CZ P10, Ruger SR9, and likely some I’m forgetting. I’d feel pretty comfortable with just about any of them.
I still prefer Glocks in overall size and my accuracy with them, especially at speed. Ive owned the 19, 23, 26, and 43 (I did not like the 43 at all). I’ve often found that differences in triggers that are easily discernible in dry fire don’t always materialize to the same extent in live fire, and frankly the Gen 5 Glock trigger is decent IMO. You may also find another pistol fits your hand better and again that’s something I’ve found to not always materialize in shooting, but that’s just me. Don’t exclude other options just because.
As for modifications, a set of sights that aren’t polymer, some more magazines (I like 5 per pistol with 3 for the range and 2 for carry), and ammunition if you can find it and afford it.
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