EDIT: Hopefully this isn't too TL;DR. I got a bit carried away writing this response.

Debian Stable uses an LTS kernel, so while it's based on an older codebase, it still gets regular security updates. You won't get the most bleeding-edge performance features, but since you're using an older Nvidia GPU, not being on the bleeding edge might actually be helpful.

I honestly wouldn't be too concerned about the space used by Flatpak dependencies, unless you have a really small SSD that you're trying not to use too much space on. If you use a separate home partition, it's helpful to set up Flathub as a user repo (add --user to the usual install command) so that the space that gets used is on your home partition and not on root.

As far as sandboxing issues go with your development workflow, I'm not super well-informed on that subject, but I know that Flatseal gives you a fair amount of control over what Flatpak applications can access.

I think if Debian Stable has one big caveat, it's that it's not the lightest distro, particularly when you're running it with a full DE and Flatpak applications.

On machines like my Thinkpad X230t or my HP Elitebook 840 G1, EndeavourOS has been noticeably snappier. However, EOS is basically Arch, and I don't like running Arch on machines I don't intend to use regularly, since it tends to break when it hasn't been updated in a long time.