Time Machine hard drive replacement, how?

I've been backing up my home and work macs since the 1980s.

Still never sure how to pick a Time Machine (TM) external HD.

Current WD My Book is failing/failed (like elderly dogs, external HDs get old and fail over a short time then quickly are dead). So I switched quickly to TM old back up HGST Touro (which I updated TM back up the moment I heard the newer one failing). But that Touro is older still and about to meet the same fate.

So, I need a new external HD for TM backup. Is there any buying advice about this to be found anywhere? What type of drive, how to size the TM external, etc. for M1 mini. I have to back up a 4 TB external but, also anything on mac so, I guess 5 TB?

Instead of getting easier, buying an external has become more complex than ever.

I could also swap out whatever is in the WD My Book, I like the form factor, if I knew how to open it!