I’ve been on Win 7 until last year (obviously up to Cubase 9 only). Apart from the FLS slot thing the only other limit was CPU/DSP.
Installed the latest Win 10 update this week that lifts the FLS slot limit and now have to train myself to get rid of the paranoid limitations I’ve conditioned myself to use 
Don’t think that Win 7 will see that feature.
However that won’t help you. Try Takashi’s FLS checker (http://planetnine.jp/wp/2018/03/25/fls-checker/) to see if it’s an FLS problem at all or something else. In case of FLS slot troubles, go Win 10 or Mac.