(Only If You want cool ).

It appears you don't want.
Why?:
1.

I located the .deb for SH3D 6.3... so I'm now running 6.3

I've never mentioned .deb package.
For more precision - You have sweethome3d 6.3+dfsg-1
and Maintainer:[email protected] so
Is NOT Puybaret Emmanuel.
2. One of my machine (at work)

Operating System: Debian GNU/Linux
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.62.0
Qt Version: 5.12.5
Kernel Version: 5.5.0-2-amd64
OS Type: 64-bit
Processors: 2 × Intel® Core™2 Duo CPU E8400 @ 3.00GHz
Memory: 3,7 GiB of RAM

And is an Dell Optiplex 780 made in 2007
It's a little bit slow but is fully functional(SH3D).

Furniture Icons in Plan option greyed out and only displays "catalog icons". I've had that problem on both the old 2-core laptop and the "new" 4-core "media machine".

And maybe more other functions You've don't discovered yet.
From my side it took me a few years to understand that.
I've write the tutorial to make others to avoid this.
Until there I'll wait for you.