2012 Mac mini erase / reinstall issues

This is becoming a saga.

I managed to reformat the drive to APFS (10.15.7). However I have not managed to successfully install any OS onto the drive, nor to restore it from the Time Machine backup.

I made an Installer SD card from a download of High Sierra I had in the Applications folder of my laptop (Intel Quad Core i7 running Ventura). I could boot the Mac mini from it, use disk utility to format the drive as APFS but got this message when I tried to install the OS:

I tried this several times, including using different thumb drives etc

I tried to restore directly from the TM backup. It was far too fast, and the computer doesn't really start up from it - something wrong there. Ideally I want to do a clean install anyway.

Then I made a new Installer using a copy of Catalina I had previously downloaded. I set it up, and left it overnight because I had other stuff to do. Upon return I got a message to say there was 3 minutes left of the install. I left it a further 2 hours, and it still was stuck with 3 minutes left. I cancelled it. Now I get this message:

...Volume contains a macOS or OS X installation which may be damaged.

In each case I follow the exact instructions from here:

Create a bootable installer for macOS - Apple Support

I cannot to my knowledge download fresh copies of the OS installers to my laptop (which is an Intel machine) because the OS is later than will run on the MacMini.

To recap - because I know this is hard to follow: I have a 2012 Mac mini with two internal SSD's. One runs El Capitan to support old Final Cut Pro projects I occasionally need to access. The other Ssd is for general usage and runs the latest OS the machine will support - Catalina in my recollection. The general use system was running incredibly slowly, and I've spent a week trouble-shooting the issue without improvement. So I reached the point of a fresh install. Which is proving far more difficult than it should be, in my opinion. Next steps?

Sorry I didn't spot your post. To check the internal SSD using the software you suggest (which I already run on my M2 Mac mini) I guess I'd have to physically remove the SSD from the older Mac mini? I'd rather not do this if possible.

Is there any way to check the SSD over a network connection, or to start the computer in "Target Mode" - the old Firewire method?

Unfortunately nothing is simple:

My attempt at reinstalling Catalina fresh from the internet stalled for hours on end. So I quit that and attempted to restore the HD from my attached TM backup instead.

I booted into Recovery mode and was asked to use Disk utility to erase the internal boot drive in question. However I don’t get an option for APSF!

I am selecting the correct drive, and have tried the ‘volume’ and the drive container (WDC WDS100…). In neither instance do I get a drop down menu offering APSF. I get options for flavors of HFS+. ExFAT etc, but no APFS. If I choose HFS+ it begins to restore the volume from the attached TM backup. But the drive was APSF before my troubles started. It’s an SSD.

A preliminary search doesn’t reveal clearly relevant advice. Help needed.

I rebooted into recovery mode and went through the steps to download the OS (El Capitan). Logged into the App Store and now I repeatedly get the message:

An unexpected error occurred while signing in.

The server gave an error during download: 403

Forbidden

A quick search isn’t producing an obvious response.

Okay this works:

Mac Erase Process has Failed - What to do? - Apple Community

Hopefully after reinstalling Catalina, and restoring from my TM backup, performance improves.

The Disk Utility is version 15.0