I'm pretty sure that a locking XP will keep a 20 from leveling even if they have banked XP. Just don't unlock it as long as you have active time. (And probably leave it locked for like a day or two after, since the system might not fully recognize you as unpaid the minute your time is up.)
What's your long term goal? Wanting to make one super twink will require a lot of investment from high level alts and/or the AH, while going for account-wide goodies mostly takes time but will require a lot of grinding to fit into one month. A few things that I'd consider important as someone who looks after account-wide unlocks;
- Complete the DF campaign, and ideally the Legion and BFA campaigns as well. Adventure mode in DF campaign is a huge bonus. Legion and BFA campaigns only unlock invasion WQs for your 20s, but those can be nice to pick up some rep or resources while they're up.
- Unlock allied races.
- Buy anything with a cost over 1k that really appeals to you. Let me strongly recommend a vendor mount here, since they're a huge QoL boost.