Well, while you may not have intended it as my solution, you did bring the idea up and explain how they worked. I didn't know you could have more than one "user" in a Chromium browser of any flavor, certainly not that more than one could be open at the same time. I assumed it was more like Netflix, where in order to use one profile, you had to close out another.
From there, I was able to mold it to my needs, and so far (one day, to be fair), I haven't continued to experience the same issues.
My current theory, based on your thought that "every profile degrades eventually, you can't avoid it" is that in fact, that was at least part of the issue, and the other part was the recent update making it worse.
I now suspect that each of these profiles will have less taxing them, since they will each have fewer tabs and will also get used less, only when needed. And they nicely solve the problems I've had otherwise with workspaces.
My best guess is that the profile I use for research, which has the most tabs in it, will be the first to degrade. But now I believe I can more quickly create a new profile, so in future it may not be as bad.
In any case, intended or not, it led me to what I suspect will keep me enjoying Vivaldi at least for a bit, and may help others who open a bucketload of tabs.
