I tried it. I didn’t like/enjoy it.

From your description, to replace LoL with a more populated game so it has a broader player base, it’d be a fit. It’s free to play, so why not try it out?

For me,

• gameplay/mechanics: generic, like all the other titles
• guidance: bad, confusing
• matchups/difficulty: genre problem of diverse characters and abilities, and at times you don’t know what’s happening. Diverse skill levels, and team matchups. Sometimes your team is getting stomped and you have no idea how or even if you could have influence on that. (I switched class and it got even worse.)
• polish/UI/UX: lacking, unoptimized, at times confusing

Honestly, if you want a personal, subjective recommendation, I would recommend Overwatch over Marvel Rivals. It’s easier to get into, also still popular, also free to play. Alternatively, SMITE, which could be considered a middle ground between top down MOBA like LoL and dynamic hero shooters. I played SMITE for a long time, but have not for a while now. Both titles keep shoving pay-content on your screen, but you can ignore all of them.

Have you played Awesomenauts?