Yup I am zeroing it.

That explains the declining speed. Thanks a lot for the insight!

It's connected over USB 3, it's SMR

Yes I am overwriting it with zeroes while formatting (using gnome-disks). I didn't know it did multiple overwrites, thought it did 1 pass overwrite with zeros.

Stopped gnome-disks format at 78% (~6 hours remaining).
Used #sudo dd if=/dev/zero of=/dev/sda1 bs=1M seek=1001250 status=progress, write speed is ~100 MB/s.
Thanks a ton!

Luckily, I got really exhausted and frustated, so hopefully it ended for now.
I'll do mostly browsing, programming, some servers always running in the background like jellyfin.
Then I'll be playing single player games on 2k ultrawide. Thinking all AMD for now, I'll consider nvidia for path tracing for the next upgrade, but don't need ray tracing for now.
Would like to reasonably futureproof it.

This is not recommended for USB connected drives

Thanks, great advice.

forgot to answer the second point, moving would be great, laptop + egpu would work, would help while travelling. But a desktop sounds more interesting without the comprosises, could get a weaker laptop for travel if needed I guess.

Hmm, I didn't even consider it, but could work out for me as well, at least for a while, as I already have a laptop which works really well, just the gpu is what really needed an upgrade.
@OwlYaYeet please help me with these doubts-

1. how is steamOS for privacy (steam stuff), I it possible to stop all steam related activity.
2. How easy it is to setup lutris/heroic to play my gog games? (compared to setting it up on regular arch let's say)