Here is my list. Would be interested to read how often others are checking various items and intervals for some of the more infrequent changes, like brake seals.
With each drive, I check coolant, engine oil and oil in strombergs.
I periodically check clutch and brake fluid levels. I replace clutch (slave and master) seals when I start losing fluid consistently. Slave goes more frequently than master. You can tell its going as the oil gets darker and I get smoke on start-up from fluid leaking onto exhaust. I also periodically check tire pressure.
Every season, I top up trunnions, steering rack with grease (some use oil).
I replace engine oil every couple of years or couple thousand miles. I top up gear box at the same time, as I seem to drip from there. I also have no confidence that I am filling to the right level, as it seems to go in and go in, and not come out, then come flooding out, and not sure how much remained in. I just did differential oil for the first time in my ownership of the car (11 years).
It wasn't that bad now that I have an access port in the boot.
On alternating years, I change brake and clutch fluid. I check suspension to ensure that everything is tightened to spec and bushings are OK during these changes.
I did brake seals the other year, not because they needed it, but because I had never done it before. I replaced pistons with stainless. I am not sure how often they need to be done. Every 5 years, I am guessing.
Hope that helps. Dan