Selfhosted

Along with the things others have said (Backups, Linux, Docker, Networking) I'd also recommend getting comfortable with server and network security. A lot of this is wrapped up in the simple mantra "install your goddamn updates!" But, there is more to it than that. For example, if you go with Nextcloud, read through their hardening guide and seriously consider implementing all of the recommendation. Also think through how you intend to manage both the server and instance. If this is all local, then it is easier as you can keep SSH access to the server firewalled off from the internet. If you host part of your stuff "in the cloud", you'll want to start looking at limiting down access and using keys to login (which is good practice for all situations). Also, never use default credentials. You may also want to familiarize yourself with the logs provided by the applications and maybe setup some monitoring around them. I personally run Nextcloud and I feed all my logs into Splunk (you can run a free instance in a docker container). I have a number of dashboards I look at every morning to keep an eye on things. E.g. Failed/successful logins, traffic sources, URI requests, file access, etc. If your server is attached to the internet it will be under attack constantly. Fail2Ban on my wireguard container banned 112 IP addresses over the last 24 hours, for 3 failed attempts to login via SSH. Less commonly, attackers try to log in to my Nextcloud instance. And my WordPress site is under constant attack. If you choose to run Wordpress, be very careful about the plugins you choose to install, and then keep them up to date. Wordpress itself is reasonably secure, the plugins are a shit-show and worse when they aren't kept up to date.