The Magura Royal Blood mineral oil works fine and mixes with the Shimano stuff okay, some say gives better performance due to lower viscosity.
You’ll need replacement olives and inserts as you can’t realisically get the old ones off and re-use them. My brakes came with spares.
SLX bleed really easily from the top in my experience without the need for syringes etc, the hardest part was finding a 7mm spanner. Just keep the reservoir topped up, it goes down pretty quick when you’re pumping the lever. Check out the excellent instructions on the Shimano website for some good tips, it’s dead easy.
Car washer hose is pretty much the right diameter to go on the nipple really securely, and you can get it at any decent car accessory shop.
I have an old set of bars I stick in my workstand, and a brake mount screwed to the leg of my workbench. Saves getting brake fluid all over the bike.
SLX are great brakes.