I second this wholeheartedly. I've just switched to Vivaldi from Chrome, and am overall extremely pleased with it. The tag stacking is a game-changer and the idea of workspaces is really appealing... However, the lack of ability to save those workspaces individually to open again later (closing in order to save memory) is a challenge for me.
On Chrome, I really came to rely on tabxpert which saves each window as an individual session that can be reopened after closed. Each session can be tailor named and it keeps track of changes to that window automatically, in real time. While that would be my absolute ideal request, adding functionality to save individual workspaces as a session which then over-writes itself when re-saved as outlined above would be 80%, and I'd happily take it!
I did import tabxpert into Vivaldi, but it doesn't work with tag stacking and with workspaces it can't track the switching of them within one window... and I enjoy these functions, so I'd love to have a native solution rather than give them up to keep using the extension.
For reference, I have something like 100 different window session and nearly 10k tabs, so it's just completely infeasible to have them all as workspaces...