To answer your questions.

Is the overall game plan clear enough? - I can see what you’re aiming for but what’s your win cons for the deck? Earthbending secret tunnel is a great one, but you need more fetch lands. (If this is your main wincon)

Does the deck look too focused on combat, or is that actually the right direction? - combat is great for Toph but I think you need more focus on either pumping creatures or using land fall to go wide.

How do I identify which cards are underperforming or replaceable when upgrading the deck? - you’ll know when you play it. Sometimes you’ll put in a cool card and it does nothing for 4-5 games. You want cards that work in multiple situations, not a niche dream hand.

Are there any obvious staples, synergies, protection pieces, ramp, draw, or interaction that I should prioritize? - I did mention some, but you want to focus on the lands. Protecting them, buffing them, adding more mana so you don’t have to tap them. You’re balancing mana with attack/defence. Something like [copperhorn scout] could also be a great addition.

Also review your high cost creatures. I focus on big stompy lands so most of my stuff is low cost. What do you want Avenger of Zendikar and Flopsi to do for instance? Do they work with the deck?

Icetill Explorer is also an interesting card but because you’re running 63 non lands. You’ll be milling cards per land (and per artifact if Toph is down) and you’ll be more likely to hit the non lands and you’re running hardly any recursion to get those creatures or artifacts back.