Yeah those ideas could definitely work. I'll have to give some more thought to the 2 xp skill buy as a down payment on a full skill. For some reason I'm not a huge fan of that idea, but I can't put my finger on why. I guess I'm just not sure if the 2 xp skills are worth putting work into saving. Maybe it should just go the way of the dodo. 4 xp really isn't that much in this game. My players usually get at least that much each session, so it's not that hard to get a full skill. The only real limiting factor it seems like is that you can only buy one skill per tier, so the 2 xp skills might be nice for people who want more skills to get past that restriction.
From my experience as a player, I've never felt I need more than a handful of skills (the few I get from the system as is). In the parties I've played in, PCs are fairly specialized. One PC makes the numenera rolls, one PC makes the perception rolls, one PC makes the social rolls, etc. With the Effort system, you really only need skills in rolls you make all the time, and with the characters I've played I've found that to be only two or three skills anyway.
After a year in the system, it's become obvious to me that every player I've encountered is resistant to spending xp for non-advancement. This is a real thing. If you want to entice players to spend it for other things, they have to be convinced it's worth it. Spending 1 xp for a reroll has proved to be worth it, and pretty much every player I've encountered eventually ends up using xp for that. The 2 xp uses just don't seem to be worth it, because I hardly ever see anyone using them. I guess that's fine. They're there for the fringe cases where it might be advantageous for a player to use. I just no longer feel it should be the GM's job or concern to try to get players to spend xp on non-advancement. This isn't D&D. In a game where you can have a tier 1 character and a tier 4 character in the same party and it doesn't matter, I really don't think it matters how quickly the PCs tier up.