If you know how to use containers then go for debian as the server OS and run everything you need as containers.

Debian is community based distro, very stable, major updates every few years, ubuntu and other distros use it as a base.

Get familiar with working on the command line, try nano as an easy text editor und use docker compose.

I did not test TrueNAS, I do know the scale version can run containers too, but I do not know how stable ur usable it is. You should go for the classic linux approach with Debian. Much more fun to learn :)

Debian

1. Host family media through Jellyfin, etc. This would include tv, music, and possibly books as well. Many of these will be managed through the Arr apps.
2. Degoogle my phone - I'm beginning by replacing Photos with Immich, but hope to also use Home Assistant, backup other phone data such as messages media, shopping lists, etc. I hope to replace Google storage/backup with Proton Drive.

Seems like a solid plan. I used Nextcloud as a Google Drive/Photos/Calander/Lists replacement, but depending on what you are running as your server it may be a bit too heavy.

I’ve heard that once you get into it, Linux distros like Ubuntu are not very user friendly for self-hosting as a beginner.

Not sure who is saying this. Granted, if your not used to *nix, our ways of doing things can be a bit obtuse from time to time but the Ubuntu based distros are some of the most heavily documented distros available with only Arch being better documented (Don't use Arch unless you've got stock in Bayer. /joking, kinda).

Your current OS choice is maybe not what I would choose, but it is fine. Xubuntu just Ubuntu Server with the XFCE desktop installed. A bit heavy for a server install as a result. One thing I will say is that most server software is setup via the command line and setup via configuration files (These are just regular text files readable by any text editor. I like micro, but anything will work). The desktop environment is just extra weight you don't really need in this application. Doesn't hurt anything, just heavy and not really needed.

So is it better on the whole for a beginner to have a popular distro with lots if documentation and step by step guides, or to have a purpose-built OS like TrueNAS that might be more straightforward, but with less support?

Stick with what you have. Ubuntu is a very well supported server distro, and the XFCE desktop doesn't change that. Things like TrueNAS, UnRAID, and whatever is the flavor of the week tend to cover things up to simplify things. This is fine when they work correctly, the problem comes when things inevitably break. You won't know where to go looking to fix things. Also TrueNAS is a network storage OS, not really suited for what you are trying to do currently, which seems to be hosting services.

It seems to be working well, but I’ve had a few hiccups trying to update it,

What hiccups were you running into? And were you using the GUI tool or apt on the command line?

On a PC? Anything except ChromeOS or Android, I suppose.

I like CentOS, or rather Alma or Rocky now. Very stable, very easy to manage, lots of community support.

Xubuntu is more than fine. Tbh it doesn’t hugely matter which distro you use for this type of thing

I'm a beginner currently doing exactly the same thing, plus a but more.

For the server I'm using Proxmox as a hypervisor, which let's me experiment with various OS's as I choose. For my phone, I have a pixel with GrapheneOS, which I love.

What are you trying to do? Debian is pretty solid but you also could go with Rocky Linux or tons of others. Heck, you could even buy a Red hat license if you felt like it for some reason.

Install Debian 11. Manage it with Yunohost. It gives you a GUI and automates most of the configuration.

If you want Something you can set up and forget, use YUNOHOST, otherwise learn how to use docker containers and pick any server distro. Debian, Fedora Server, Ubuntu Server, MicroOS, hell OpenWrt will all do fine.