I also have an iMac 2011 2.5ghz 16gb Ram / 1TB SSD and the only issue with it is it’s stuck on High Sierra.
It’s to do with a newer graphics setup called Metal that doesn’t work on very old AMD GPU units (or sales BS).
However, someone online found out how to get a second hand Nvidia k1100m / 2100m (common in loads of old laptops) and install it and that then lets you upgrade to the newest OS with a tutorial on YouTube.
Worth doing as there’s literally nothing wrong with it for what I do.