i replaced one of the two reds with a red plus (due to a defect). as an "amateur" i did not notice any changes in performance.

If by "normal Red" you mean an old WD Red with CMR, it is absolutely worth it, as they are practically identical. The only "relevant" difference is the corrected firmware in the 2021 version.

If you don't have huge demands on your NAS performance, you can of course also do this in combination with an old WD Red with SMR.
(Personally, however, I would replace an SMR for my requirements).