I like working in native apps on my Mac for spreadsheets, so I've used Numbers a little bit, but if you have the same iCloud file open on two machines they conflict and don't merge nicely. Does this have a better story there?
Almost certainly not; it really depends on how iCloud handles the sync. Cassava supports (at the moment) CSV (and its variants TSV, etc.) and JSON files. If your sync mechanism works well for those files in a text editor like vscode or sublime, then it will work well with Cassava. If not, there's not much we can do on our end - that's really a problem with how the sync mechanism works.
Linux support is there and working, half my team uses arch btw. Windows is working, I just don't build for it right now.
Excel is cripplingly slow (thought more capable). I don't like using text editors for sheet data; it's simply not as ergonomic to browse a CSV as a text file.