What open source architecture design tool do you recommend?

Diagrams.net looks pretty reasonable.

I just use Libre Office presentation tools. I try to keep the block diagrams simple and high level.

I also use diagrams.net but I use standalone version downloaded from Microsoft Store.

Seems a bit spammy, considering that you mentioned a non-open-source tool. Why does it need to be an open source tool? There are plenty of more professional tools than the non-open-source one that you mentioned, but they are not open source either.

What's wrong with PowerPoint, or the free equivalents like Libre Office?

If you want a proper professional tool as actually used in industry, I can suggest some but they are not open source.
If it is work related, then there may be an application the organisation wishes you to use, or, sometimes they won't care whatever app you use, since the output doesn't directly drive automation in the project management (i.e. it's not directly coupled). If you want others to also be able to work with the design, then you probably want to stick to an app that others have access to. I wouldn't trust the cloud version of that app, and I can't see I'd want anything from that app that you refer to, when PowerPoint will work for me if I'm not being mandated to use any other app.

First, my software IDE is eclipse IDE. I have separate installs for C, Java, snd others

Second, for simple diagrams, I use DIA

Third, for my schematics, I rely on KiCAD.

Fourth, I have been trying to get my head around Papyrus using sysML (from the folks at eclipse) for system flows.

Fifth, for programming I really like C (GCC) (not C++) and Java (community)

Sixth,  I use LibreOffice Writer for much of my documentation, but It gets published in Correl Ventura ( I have more control).

AND Lastly, anything from MS is suspect and just trash and NOT OPEN SOURCE!!!

You do realize that LibreOffice supports MS extensions. 

I use EAGLE for diagrams and pictures other than circuits; it's got a bunch of "features" (explicit object size, polar coordinates, multiple layers, a programming language to create repeated elements) that are quite handy, and not so common in traditional drawing programs.  I imagine KICAD has similar capabilities (and many other CAD programs, too.)

The "Gnu ID Utilities" (https://www.gnu.org/software/idutils/ ) and ctags/etags are very handy for browsing unfamiliar source code.  The\y provide the sort of "find implementation of function x" capabilities on the command line, so they're agnostic to IDEs that I'm not familiar with, that the code might have used.

And EMACS, of course!

Personally, I use LibreOffice and it's pretty good.

Doug,,,, use dia.  it's in my Debian 11 Synaptic System: dia, dia-common, dia-rib-network, dia-shapes, dia2code. 

If you cant find them let me know I will send you the packages.

lol I wish MS would fix this:

https://office-watch.com/2022/make-lines-arrows-straight-word-bug-fix/ 

My (not so) secret technique for almost every diagram, work related or otherwise, is to use/misuse PowerPoint, and a benefit is that anyone can modify my diagrams since managers often want to strip off some complexity and reuse the diagrams for their high-level purposes too, and they already have Office on their PCs so they don't need to load anything special. Libre Office equivalent is presumably just as useful. 

Another excellent tool/time-saver is Snagit (I dont know what the closest Linux equivalent would be for that), which is extremely feature rich and allows me to convert almost any image into transparent PNGs for dropping into PowerPoint, to use in diagrams. There are so many uses for Snagit, and it's gone up in price unfortunately, but it's still cheap for what it does, considering I get use out of that app every day.