I'm sure this has come up before, and the eventual answer always seems to be 'there isn't one'... (Maybe you could start one..?
When I say there isn't one, I mean I've never seen a "hey, here's the top 4 or 5 useful ROM routines that a lot of people found useful when learning assembly"
There's a few 'here's the entire ROM documented' like the skoolkit one, but while that may be useful for reference, it's only useful if you already know what you're looking for. Otherwise it's a needle-in-a-haystack situation.
I reckon the best I've seen on this is the very tutorial/guide you've been looking at - from what I remember, it covers:
- clearing the screen with different PAPER, BORDER colours
- playing sound effects/music (the BEEPER sound routine)
- printing strings of characters/text using the ROM print routine
To be honest I don't think I've ever used any other ROM calls, from what I remember.