I have a collection of over 400 GB (about 70,000) family photos on a 2012 Mac mini that is my family media server, hooked to an HDTV and a sound system. I do not use iCloud Photos to sync across devices.
We are a Mac, iPhone, and iPad family, with WiFi and wired network of obsolete, vintage, and recent Macs, including a MacBook Pro 2009, MacBook Air 2015, iMac 2019, a 2021 MacBook Pro M1Pro, three iPads, and three recent iPhones.
So a few caveats:
1) If you use the Photos app to import and organize the images, as Neil22R suggests, Photos 5.0 is the only version that runs on macOS Catalina. If Photos 5.0 handles all your photos organizing and editing needs, that's great. (Photos 5 does not have a 'Find duplicates' feature that was added to Photos 8.0, for example.)
2) The newest macOS version that runs on a 2012 Mac mini is Catalina 10.15.7. Apple typically provides security updates for the THREE most recent macOS versions (Ventura, Monterey, and Big Sur), so except in very unusual circumstances such as a severe security issue, Catalina 10.15.7 will no longer receive any updates, period.
(My Mac mini nags me to install a 'required update' every time I connect my iPhone or iPad via USB to import photos, but the update always fails to install, even in Safe mode, and the photos always upload just fine anyway. I've reported the issue in these forums and filing a bug report, but no solution has been forthcoming.)
3) If you have more than one Mac, and any of them have newer versions of macOS installed, be aware that accessing that Photos 5.0 .photoslibrary file (such as sharing your .photoslibrary file across your network) with any newer version of Photos (6.0 from Big Sur, 7.0 from Monterey, or 8.0 from Ventura, respectively) will make the 5.0 photoslibrary permanently unusable by Photos 5 and that Mac mini. It will become usable only by the newest version of Photos that has accessed it.
4) You can never be too rich or have too many backups, especially when it comes to family photos.
It is impossible to recreate lost photos of children growing up, holidays, family reunions, and vacations. Plan ahead for multiple backups of your photoslibrary on multiple devices, using multiple backup strategies. I use CarbonCopy Cloner to make multiple bootable backups of my boot drives onto locally-attached hard drives for fast recovery. I use Time Machine to make ongoing backups onto separate local and network-attached storage drives. Time Machine is not a 'permanent' archive. If you delete something from Time Machine today, it will eventually be deleted as your oldest backups are replaced by newer backups, as Time Machine manages the backups using the space available on your Time Machine backup drives.
I recently discovered that a failing SSD corrupted a Photos 7 library. (It is essentially a copy of my Mac Mini Photos 5.0 library, but running in my 2019 iMac, so Photos 7.) I was able to recover all but about a dozen of the 70,000 images, and none of the ones lost were 'priceless' images, only ones of a series taken at that same time.