Really depends on what you're expecting to do. At a bare minimum a good bleed kit for your specific brakes (don't go crazy cheap, make sure it has the specific tools needed), nitrile gloves, a disc brake truing tool, blue shop towels, 70% IPA, a stand that allows you to hold the bike by the seatpost, and some soft wedge type tool to spread the pads. Also look at your manuals for any specific tools sizes you could need. I have a little 4 inch long wrench (I think 7mm) that is used for a lot of Shimano stuff that's a life saver. I also have a stash of bleed blocks and the little orange pad inserts.

If you think you'll be swapping/ shortening hoses frequently, a really good cable and housing cutter is just about a necessity. I also got the Jagwire needle driver tool which is super helpful for getting barbs pressed in.

There are also a few more small parts in the assembly that it's nice to have extras of on hand. I keep some extra bleed port caps (every bike I've gotten they've been way over torques or rounded out from the factory), pad retention screws, barbs, olives, cotter pins, and brake pads on hand. All of this is pretty specific to the model of brake you have so do a little digging through the manuals once you have a bike picked out.

Shimano or SRAM?

Bleed kit if you want to handle it all like shortening hoses, or bleeding once a year if that for a TT bike. Otherwise a basic rotor install tool (freehub lockring or bottom bracket) as well as a nice sized bottle of Isopropyl to keep those rotors clean and some extra brake pads which install easy. A tire lever can work to push the pistons in, or you can get fancy with a more specialized tool.

Mechanical or hydraulic disc brakes?

I have not decided... which ones are more prevalent for road/tt bikes?

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Next races on the schedule: none at the moment

Both.

Patience

This is by far the most important tool you need. I still hate working on disc brakes but this tool has made it almost tolerable.

^This. And YouTube.

Have had years of working on hydraulic disc brakes on mtbs and fatbikes. Now we now have several road and gravel bikes with hydraulic disc brakes. Long term complaint has been the need for multiple bleed systems between manufacturers. The additional requirement is a split between DOT brake fluid (SRAM) and mineral (Shimano). Right now we are about half and half on the fluid type. Park has full kits that are a bit pricier than OEM offerings, but cover lots of different manufacturer and model options. But they still have one for DOT and one for mineral which doubles the cost and equipment if you have the need to service both.

Something I just don't get... wht bleed brakes yearly ?
How often do peoe get you hydraulic brakes bled on you car ? Never.
(I've been using hydraulic discs on mtbs for the best part of 20 years.
Only time a bleed has been needed is when the calipers have been so fecked or stuck they've needed replacing (or with my Hope brakes then a change of pistons amd seals as parts are available).
Or if a hose got ripped out in a crash..

I think a lot of this come back to the old military maintenance addage... if its not broke, lets feck about with it until it is.

Never? I crack the bleeders every pad change and flush it every 50K.

On a TT bike, probably never needed unless you are shortening hose or installing.

On a CX or Gravel bike with heavy use, grime, and pad wear, more often.

Brush up on your curse words!