I don't want a spreadsheet in my browser...I want a spreadsheet that can sync as nicely as Google Docs though.

I'm sort of split 50-50 here. For alot (but not all) of my simple computing needs, i do see the value of (and appreciate/like) having an app in my browser...However, when we get to more powerful apps/tools like spreadsheets, that's when i really do NOT want them in a browser. And, as you noted, the holy grail is indeed having those implemented as offline-first, local, super-fast-to-respond, with locally-saved data app...which then can be configured to synch data to our other (user-controlled) places. Basically, the concept of what, say, Microsoft does with their Word app - where it is a locally-installed app, but it can be made to save locally and synched online, and then alternatively use the online version of Word (which i know, is close to horrible, etc.)...that's a close approximation to what i would liike...but, no way do i like that this is managed by an entity like Microsoft (or Google or other big providers that i do not trust).

To the original question posed by @spit-evil-olive-tips , while i have experience self-hosting the basic stuff from Nextcloud, i have never used their Office stuff (https://nextcloud.com/office/)...though i hear it can be nicely integrated with Collabora (https://www.collaboraoffice.com/). Maybe you might want to check out how the spreadsheet app works there?