Some networking basics (Firewall, VPN, NAT, DHCP, ARP, VLAN) makes every selfhosters life easier.
1b. Your ISP router probably sucks, but you might be able to experiment with some static DHCP at least. I'm a fan of the BSD based routers opnsense/pfsense but depending on what router you have you might also be able to run OpenWrt on your existing router.
Some management system and filesharing basics (NFS, SMB, SSH, SCP and SFTP).
Learning how to set up a backup for your stuff. The hypervisor you choose may or may not have a built in solution.