Both devices can be upgraded easily, settings can be kept from 21.02.0 (ipq806x hasn't migrated to DSA yet, but will soon, for the next major release), but there will be more significant differences between 19.07.x <--> 21.02.x than between 18.06.x <--> 19.07.x. This means while retaining settings over the upgrade will mostly work, downgrading won't (due to the changes in the netifd configuration syntax).
Given that the nbg6817 is a dual-firmware device (leaving your 19.07.x installation and its configurations untouched on the other partition set), I would suggest not to retain settings though.