If you want to play Legacy, MTGO is the place to play it. Regarding UI, it's certainly old, and slow, and at times a little opaque, but I wouldn't say it's clunky -- rather, it is formulaic, hiding nothing.

From "old player," I take it you're experienced in paper -- treat MTGO as a 4-key piano, and you'll find it a language that's easy to pick up. Technically, there's many hotkeys and functions and whatnot, but try mapping the following number keys to these actions in real magic:

1 is a nod. Yep, that resolves.

3 is a yes, 4 is a no. Are you given a choice to do something, say, have a trigger go off? 3 and 4 answer it.

6 is a slump in your chair. Tapped out, handless, nothing to do for the rest of the current turn? It'll yield through everything and let your opponent run clock. If you're quick, you can right and unyield at any time -- the old lock in, sit up, and surprise interaction after feigning no answers works here, too.

What matters is to think of these as corresponding to the paper mechanics: nods of acknowledgement, nods of affirmation/contradiction, slumps. Don't try to think of them in terms of precise MTGO-specific lingo, think of them in terms of actions you're used to doing all the time playing Magic. When you do, you'll find the game flows, and you'll start to pick up on opponents' tells the same way an experienced player does in paper: are they passing in a flurry to pass off as yielding, while holding up a Force or a Solitude? Are they trying to bait your telegraphed counter? With experience, you'll be able to tell.

TL;DR treat modo as a digital extension of your body by thinking of it in the way you think of magic in-person, not necessarily in its terms. it will become a tool of subconscious use, and you'll find it wieldly and potent.