I’m quite forgetful so don’t want to need to rely on keeping my phone charged in order to be able to log onto my Nextcloud (at uni so jump between computers a lot), but I want to have 2FA enabled. I was considering having both U2F and TOTP enabled (so I could use either depending on what was easiest) is this possible, if so how?
It’s possible and I do it. Just enable U2F and TOTP for that user and then you can pick which one you want at login. I have it set for one of my user accounts. Technically, having both enabled is a bit counter productive to security. Namely, a person could still steal your TOTP value or just get their hands on your key. Plus, in a way TOTP reduces the security of you U2F key because they can now just focus on your TOTP login to get in, and not getting your key. But this is all really theoretical security and based on the risk of an attacker trying to target one or the other being slim, it won’t be a problem 
just sharing that tidbit of info for the heck of it.