SketchUp 2021 is the only version that is stated to work with Big Sur. That means that if something comes up in Big Sur that stops 2021 from working, we would do something to 2021 to work around the problem. We won’t go back even to fix 2020, let alone 2018.
But, that’s quite a different thing from whether 2018 will work ok in Big Sur. I switched to Big Sur a long time ago, and I know that at least one of the developers was on it before I was. What I’ve found is that 2016 closes after not too long, 2017 can last quite a while before it might quit, and I don’t think I’ve seen 2018 quit on its own.
I am now on Monterey (macOS 12), and the situation is roughly the same. I don’t currently have 2016 installed, but 2017 and later are the same as they were on Big Sur.
I’m pretty sure that 2018 will work at least well enough for you to use when importing difficult DWG files.
Would it be possible to try one of the files that fails, so I can see if the problem can be fixed in 2021?