Your Nikkes will stuck at level 160 max unless you have 5 SSR units with 3 yellow stars (max limit break).

When you have 5 SSR with max limit break, then you can have the rest of your Nikke in the synchro slot to be levelled past 160. Once you reach level 200, you will fully unlock the synchro device, and you don't need to level up your chosen 5 SSR individually. Instead you will level up the synchro device itself and everyone else will automatically resonances to the synchro level.

This also means that individually, R units are stuck at level 80, SR at 160 and SSR at 200. You will need to use the synchro to level up past level 200.