Firefox would give me weird problems with teams in the past. Have not tried it in a while though. I'll try it and if it works without problems now I'm happy to leave chromium behind.
For teams specifically I'm just using the web version with chromium. Installed as a chromium app so I have quick access to it and have it on my taskbar. Rest of Office 365 works just fine in any browser. (Outlook, SharePoint, Power apps etc) For OneDrive Sync you can use https://abraunegg.github.io/ which should work on most distros.
Since you using the software documentation and creating issues you find would be a good start. Also as a sysadmin you might be familiar with integration of applications into other software which could be useful as a guide for other users. Especially scripting automation for your spefic deployment.
Download office from massgrave.dev and activate with the power shell activator. Permanent activation as easy as it gets.
Take a look at simple Android TV boxes with a USB port. There are plenty of options and you can just install vlc on them
Get a used Thinkpad. Shop around eBay for a T480 or T490 which should be at that price range. Solid machines with great Linux compatibility. Anything new will be much worse at that price point. If you desperately searching for something new maybe a HP 255 G9 with a Rzyen 3 would be fitting. Not as good built quality wise and I'm not sure about Linux compatibility but at least it is upgradable. (https://h20195.www2.hp.com/v2/GetDocument.aspx?docname=c08017466)
You'll need some german trackers. PM me and I'll send you an invite for one to get started.