To add on to Patty's answer, I've found that Creeper is a great easy map to grind and that 3Rainbows is a good medium map.
Droppers are also criminally underrated, Pickles can be completed in under 10 seconds per try and if you're very efficient, copy/pasting commands and perfectly setting checkpoints, you can get 50+ mana per minute (if I remember the mana payout correctly).
Lastly, the champion maps are likely the best way to gain mana. If you're at least Skilled rank, you can get 1,000-3,000 mana every 2 days by completing the champion maps Winter Wonderland, Haunted Manor, and Aquatic Adventure.
Aquatic Adventure is very challenging and unforgiving as checkpoints are sparse and the stretches of parkour are long; I haven't been able to complete this map thus far.
Haunted Manor is quite good, but relatively confusing; there are several Youtube tutorials that can help you navigate the map, which takes roughly half an hour to complete (for me, anyways).
Winter Wonderland, though, is by far my favorite out of all of these. I'm partial to the winter-themed build, but the parkour is relaxing and simple for the most part, and you can complete the map in just 10-20 minutes with enough precision and skill. Good luck with grinding!