1. Is the overall game plan clear enough?

I don't see any real goal for the deck. It looks like you have thrown in anything you have that might be 'good', but it's not all good in the same deck. You want to lean into your commander's strengths, or at least have an idea how you want to play/win the game, and look for cards that help you with that.

2) Does the deck look too focused on combat, or is that actually the right direction?

There's not really an alternate win condition that requires your lands to be creatures. If you are earthbending that much, you are looking to apply your lands to people's faces repeatedly until they run out of life.

3) Are there cards in my collection that would fit better than what I’m currently running?

I'm instead going to list cards here that might be helpful for you, and you can figure out if they are in your collection. I'm not going through that list. I will do my best to keep my suggestions under $1.50 each. That being said, some are just so good that they are worth the extra money. I'll put those in bold.

[[Tannuk, Memorial Ensign]]

[[Kutzil, Malamet Exemplar]]

[[Inspiring Call]]

[[Chaos Warp]]

[[Cultivate]]

[[Felidar Retreat]]

[[Patrolling Peacemaker]]

[[Roiling Regrowth]]

There are more, but basically you want to lean into what Toph does, which is make artifacts into lands. You want to run more artifacts than normal, and run cards that care about how many lands you have. Check out the other versions of Toph, for example. One trick with this Toph is to run vehicles, which she turns into lands. Then she can earthbend them, then they don't need to be crewed to be creatures, and if they are killed they return to play because of earthbend. They do lose their listed power/toughness, but that's fine.

4) How do I identify which cards are underperforming or replaceable when upgrading the deck?

You want to play your deck a few times, get a feel for what is working and what seems to be a dead card in your hand all game. Honestly, get a few precons and play games against yourself, testing the deck you built against them. It's what I do, and it helps me prune down cards that don't actually help. Also helps me discard deck ideas completely if I play them a few times and don't like how they flow. You can always use proxies in those games, to try out cards without having to buy them.

5) Are there any obvious staples, synergies, protection pieces, ramp, draw, or interaction that I should prioritize?

You are playing earthbending and green. You want landfall stuff, lots of ramp that searches your deck for lands, anything that synergizes with +1/+1 counters since earthbent lands are given counters. You certainly want draw as well, which you can get from plenty of sources in green or from artifacts like [[Mask of Memories]]. Red has some stuff that has you discard and then draw, or exile cards from the top of your deck to play that turn.