substitutes for crop rotation?

In standard gardening books, brassicas are listed as some of the most important crops to rotate. I will have difficulty doing this, however. I only have a handful of beds that are suitable for brassicas in my home garden, and due to food preferences they are all planted with brassicas this year. (Brassicas here need row cover, which means that small irregular beds among other, perennial plants are not suitable.)

What can I do to minimize disease and insect carry-over? Solarization? Cover crops? Etc.

I would probably plant some other crops interplanted with them. Maybe something strong scented like onions mint.

I would be very careful to pick up and remove fallen plant debris from the beds as a first step. Cover cropping is a good idea. If you find you're having disease problems after a couple of seasons, you might consider sheet mulching the beds at the end of the year and effectively starting with a new layer of topsoil, but that's a lot of inputs.

Cover cropping is a hood step I reckon. I'm not super familiar with the particular problems that might build up with brassicas but I reckon that they are organisms that serve as food for something else so finding the predators of your pest and inoculating with them and facilitating their continued residency that should help. I Think that in general, minimizing soil disturbance and keeping living roots going all year is gonna be your best defence. You may find that you need to take more proactive steps, like adding some kind of foliar spray schedule, to stay ahead of problems

Many smart people say if you get the biology right (quality and quantity) that you can crop year after year without disease or yield decay.

This guy is probably a good place to start  https://www.advancingecoag.com/