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

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

I need it to be completely without fans due to the location where I need it.

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

+1 for Joplin. Works really great and has some good plugins if you need to extend it.

Nextcloud

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

If Cron is too hard for you (it is really easy so I don’t really understand your problems) then check out runat instead. Simple and easy to use with all the features I would suspect you use in cron

I would go selfhosted and use a mai-in-a-box solution. Easy to setup, handles all the spam DNS settings etc and if you put it on an ip segment that is generally spammer-free, then you should have a fantastic solution (I habe used it myself for many years).