Buy an external hard drive or SSD, download the photos to it, put it in a safe place away from your other computer stuff.
While I do keep a single drive backup, to restore my entire drive from, but I do not test this backup. If it doesn't work, it's not that big of a deal. I cherry pick what I really find important and burn it to M-DISCs. Very little content is actually that important, baby photos and things like this.
They both have an app you run in your phone, and a companion app you run on macOS, Windows, Linux, or your NAS, and it syncs (even through a NAT) whatever you've configured to backup on your phone to N servers.
If you're away from home, you can run it on your laptop next to you, so your phone is backed up quickly, and then throughout the day you laptop can sync with your home server while you're out exploring. It's nice.
I have two backups online, and an external drive I rsync to monthly then turn off.
Before you delete your originals in Google Photo, use Takeout to validate the files. Mine (at "original quality") were mangled.
- I recommend Syncthing (easy) or Seafile (more features).
Both are free for personal use but the latter is a product (paid plans, support), Syncthing is more of a 'project' (both work great).
You can self-host the recipient machine at home or on some cloud (depends on your internet connection, needs, skills, and budget, I guess).
[IMHO it's a must-have skill in our historical era to be able to manage data at a basic level, just like managing one's finances, or health status; so I recommend learning 'enough' about it.]
- "Data that only exists once (1 original copy) does not exist." — the idea of a single point of failure without possible recovery.
Have at least 3 copies: 1 original (e.g. phone, laptop); 1 'repository' (file server) which aggregates all sources and becomes your central source of authority; 1 backup. (this ensure that you never go below 2 copies, if any 1 fails). I use Google Photos too in addition to Syncthing — sort of a failsafe for uploading.
- For long-term storage (photos usually qualify), consider bit rot — the fact that storage devices simply fail partially randomly, i.e. you lose literal bits here and there from time to time, unless precautions are taken (e.g. self-healing ZFS array).
This makes cloud services usually attractive, they take care of all the low-level behind-the-scenes shenanigans. Conversely, long-term physical backups (HDD) should periodically be either fully checked or rewritten — but they're far cheaper, now below $25/TB, so it's easy to just have two backups, one 'hot' (disk online) and 1 'cold' (disk in a dry closet). I would recommend a filesystem with checksums.
If you need more advice or guidance, feel free to ask questions.