Actually, you can still get Sierra here:

https://support.apple.com/en-us/HT208202

I was in the exact same boat as you, running El Capitan for as long as I could before Logic demanded upgrading. So last night I bit the bullet, upgraded to Sierra, and installed LPX 10.4.



Ron



UltraPAK 2021, 27" late 2012 iMac, 16 GB RAM, OSX 10.15.7
Logic Pro X 10.6.3