I came into Debian with that philosophy as well, but I eventually gave up on all native packages as I got tired of having to deal with the rotation of some testing packages.
Honestly, 1 GB is an extreme it could get to, but most don’t because the majority of that initial 1 GB overhead is shared with other applications. Part of this is design issues in glibc preventing reverse compatibility with older glibc applications, and so applications need to have the right version of glibc with them to work. This adds some overhead, but is mitigated because many Flatpaks use the same glibc version.
Also, to be honest, storage is cheap these days, and really, I think the ease of Flatpak is worth what becomes a pretty minor storage sacrifice.
Usually, Flatpaks. My generally philosophy is that if it isn’t in Debian, it probably won’t last. I make exceptions when something is the best tool for the job, like Tom J Watson’s Emote.
This isn’t not rock solid, I admit - there are plenty of defunct projects that were once in Debian repos (neofetch is still in sid), and there are plenty of lasting projects outside Debian.)