Many thanks to everyone who has replied with various suggestions. I have read them all carefully. So many options, but none of them the perfect solution (just like life really).
As I have just loaded about 100 CDs to my MP3 player, it will take me a while to listen to all of that, so there is no rush.
I just want the chart-toppers, the classics, the bangers, the choons, the pick of the pops, etc., from the last 7 or 8 decades. Add in the best of reggae, blues, jazz, folk, classical, etc. (I have eclectic tastes), and that is probably only a few tens of thousands of songs, maybe a few hundred thousand at most. I don't need 10 million songs, of which about 95% are mediocre at best. Nor do I need to explore new artists.
Say Spotify costs about the same as a CD each month, say £10 for ease. What if the platform goes bust after 20 years? Or I can no longer afford it? (Don't think it could never happen). I will have paid £2,400, and have nothing to show for it except the memory of 20 years of listening. If I buy CDs or online tracks, I could have 240 CD's worth, or about 3600 tracks, mine to keep.
I like the idea of buying a second-hand device to plug into the stereo that has internet capability - to stream music into the full-size speakers. Not as good as ownership, but maybe a good compromise.