I agree with everything except for the Ubuntu part. I rather use a lighter Linux distribution that takes freedom and security seriously, or better yet, use NetBSD or OpenBSD instead.
I‘m using debian without all this GUI crap. Should be also fine, shouldn’t it?
Not for me, really. I used Debian for 23 or 24 years, until they decided to to stop giving me the freedom to chose components and go with systemd.
I upgraded from Debian ascii to Devuan ascii and have been using Devuan in servers, desktops, and laptops since then.
If you already like Debian, you'll certainly like Devuan, since it is actually the continuation of Debian but without systemd.
I currently run a prune BTC node and a full LTC node on a Pentium 4 desktop with 6GB RAM and two 512GB HDDs on BTRFS RAID-1 on Devuan chimaera. I got all the hardware for free, and it works very well.
I want to run a full node on this box (I do in others). so I'm planning to replace the HDDs with 1TB ones and rescan the BTC blockchain. :-)
No, nodes don't need to run 24/7. For a while I've been running some nodes only at night and during the weekends, when electricity was cheaper.
What happens if I do some transactions, the switch it off and switch on in a few days?
Your wallet will still have your private keys, ready to sign transactions to expend your utxos. The node will synchronize the blockchain when you turn it back on. If it's been just a few days off, synchronization won't take too long.
What I usually do is having small amounts on LN or on-chain wallets on my phones ready for quick small transactions while my nodes are off.
I'm glad I could help. It's all a little confusing at first, although not complicated.
Perhaps you could start running a testnet Bitcoin node. Synchronization takes much shorter, disk space is much less, and stress for the possibility of making a mistake is also reduced a lot.
Also, it's cool helping run the testnet and you can try any crazy thing you might think about. :-)
In order for other peers to connect to your node, you should run behind tor as this allows inbound connections. But most "batteries inlcuded" nodes do that anyway. Run umbrel (I'm using citadel, a FOSS clone) together with electrum server and you're ready to go. Wallets can connect via onion address to your electrum server. You don't need to open any port on your router for you home network.
Your node, your rules. And your node doesn't have to be online 24/7 (unless you plan to run a lightning node on top of the bitcoin node). I run my node on a VM as guest of a work-horse desktop PC (which is actually online most of the time anyway).
The real benefits come when you transact with your node and you connect your wallets to it, it gives you better privacy.
I'm not a big fan of that raspberry pi cheap hardware crap and ecosystem. Better use a old laptop and a ubuntu LTS OS and keep it updated.