> Is concurrency in this sense mostly just 'how many terraform plan/apply runs you have going at once?'
Yes, though not by minute, but by "max concurrency" over a month, with some room for bursts.
> Also, is the Enterprise plan significantly more expensive than the cloud plan?
It's quite a bit more expensive. I do recommend contacting our sales team[0] and presenting your use-case, though, to get more details. You can definitely work something out with them.