I'd get a longer gun, it teaches you not to squish yourself against corners
I all honesty, you got some good tips here, I'll just say stay light, cqb is where speed is really the key to victory, a proper high speed, low drag loadout will help you a lot (along with a carbine size gun imo, not those silly 10cm guns), so a decent but light PC would be a great combo of protection and real estate, get a few bangers (trmr with multi shot base), a secondary and you're set.
Boots over trainers, nowadays you can get some really lightweight boots at a reasonable price (if you only play indoor I'd not care about them being waterproof tbf, a good pair of summer boots will keep your feet dry from sweat and weight as much as your sneakers).
Cover your skin but don't end up like the Michelin guy, you'll overheat In a second and have a bad day..