I couldn't have the root ball any lower as the soil and sides of pot would be pushing up against the lower leaves and rosettes a lot.

So the roots at the top of the root ball are exposed to the air and strong sunlight. I'm guessing this isn't a good thing?

I'm thinking I should just try and build up some soil around it?

That's not good. Remove the other plant and seat it higher, then fill to the top of the container with soil? Remove the offsets to allow the Echeveria to sit slightly lower? (Won't solve the problem but might help with the other solution.)

I took it out, dug a deeper hole, and luckily I managed to get the root ball down far enough without it feeling like there's so much pressure on the rosettes that it could damage them, and I then made the soil level a bit higher. So now all of the root ball is in the soil.

Looks good!