Looking to replace my QNAP TS-473A unit? What would you recommend?

Looking upgrade from 4 drive bays to 6. Would be good if the new NAS has upgradeable RAM as I run VM's, and would be great if the new unit had M.2 slots. I am also wanting pcie slots, 2 slots would be good. Dont mind if the new NAS was rack or stand alone. I have previously had QNAP NAS that have upgradable CPU's, but would only explore that if the price was low. Any advice or suggestions appreciated.