There's quite a few holster options for IWB that are 'tuckable', meaning you can wear the holster IWB, then tuck your polo or collared shirt in over/around it. This leaves only the belt clip of the holster visible, which is generally innocuous looking (or, you can let a little bit of shirt material hang over the clip making it invisible).
I prefer AIWB for accessibility, but you can also tuck in at 4:30~5:00 position.
I'm a big fan of Crossbreed holsters, but I'm sure others can vouch for other manufacturers that are also tuckable.
These are both great guns. I almost exclusively carry a Glock, but the 365XL is definitely easier to conceal. A point to consider, however, is that the 365XL's grip is smaller & narrower that the Glock 19. If the Glock 19 fits your hands well, the 365XL may take a bit to get used to. A common issue I've seen is that when the shooter's hands are too big for a gun, their strongside fingers tend to wrap too far around the grip, putting pressure on the inside grip panel, which results in shots stringing inboard ((ie, a right handed shooter tends to pull shots to the 9 o'clock))..
Still an excellent gun, & the trigger is Sig-nificantly nice! The grip issue isn't insurmountable, but I'd recommend dry & live firing one to see if it fits your hand size well.