Slack is crap and matter most has mimicked all its worst parts.

Lua is used as built-in language in more and more apps. I use it on a daily basis for configuring and extending neovim

I need it for normal desktop use. Email, coding, browsing (no gaming)

Yes i would say 1000-1200euro tops for the budget with option to add more ram later. Would you think i3 is enough? I am not too familiar with the newer generations

Nextcloud

Switched a long time ago - it’s just soooo much easier and I never have any problems