I've used quite a few holsters for IWB wtih the P365.
For me, my order of least favorite to most favorite are:
Harry's Holster IWB (did not like. Rough on inside, too many mounting protrusions since it's so modular, and retention was not good)
Vedder Light Tuck (very decent holster at a decent price. FANTASTIC mounting clip. Wish the front side came up a bit more)
Wingman Holsters IWB (AMAZING holster shell, perfect fit/retention. Hardware sucks, but swapped it out for a Vedder clip. Now amazing)
Axis Slim (most expensive holster I've ever had, and now I see you DO get what you pay for. Retention is a bit on the light side, but the fit/form/comfort/concealment is unrivaled. Absolute best there is... but you'll pay for it. Bonus for T1C NOT using the retention screws as an anchor point for the clips or claw, but that's their excellence showing there, as they made the effort to use SEPARATE mounting points for everything so it does NOT affect retention and is rock solid).
ETA: for coat pocket carry, I'm LOVING the Vedder "pocket locker with thumb push tab". Amazing. You grip the gun, push off with your shooting thumb as your fingers pull the grip. Pops right out, holster stays put in your pocket. Well made, and fits like a glove.
For a mag holster, use a Safariland mag pouch/holster (the newer accordian style. They run TIGHT on regular double stacks, so they fit the 365 mags perfect).
Either that, or since the mags are small enough, just use a multitool pouch.