Most of the key stuff has already been said
@HamboCairns get Pedros Cassette pliers. I used to use chain whips and they were the fucking anti-christ.
cassette pliers are the way. no more hand injuries because the chain whip slipped or some BS like that.
you clamp on to the cassette with the pliers, and pop the cassette removal tool in, a bit of appropriate force, and off comes the cassette lockring.
pedros.com
This, I finally got a pair of cassette pliers & love them over the previous chain whips.
Also, if you happen to have RF Aeffect cranks, like I do, then make sure you invest in a big fucking torque wrench, like one for cars. 61Nm of torque to get those pricks on (and off) properly.
Torx set, in particular the T25, as has been mentioned is a particularly handy tool to have around and not that expensive.
Chain link pliers are one I value having around a lot too, saves a lot of swearing trying to undo a stubborn link or install a chain.
Some Isopropyl alcohol is also handy. I got it initially for shock servicing, but it can be very handy to install grips & spot cleaning, rotor cleaning, etc.
Aldi bike stands are also a steal if you wait till they're in. Not the flashest but they do the job and are cheap cheap cheap.
A pack of nitrile gloves, saves me riding into works with filthy hands after I've forgotten to fix something the previous afternoon, woken up and got in my bike gear, ready to go only to find out I didn't do the thing I needed to.
The other thing I've found really handy in the long term (although more investment up front) is workshop quantities of cable outer, brake mineral oil, grease & other fluids (for shocks). Always there when you need it and more economical in bulk.
Most of all though: Patience and resolve. Especially in the sight of bloodied fingers when they've been stuck in a spinning rotor.