@pkreuzt - I have been meaning to upgrade my MR3020 to OpenWRT 18.06.1 from LEDE 17.01.4 as well (just haven't had the time). RAM could still be an issue, but the primary problem is the 4MB flash.
With that said, my recommendation would be to only perform the upgrade if you either plan to use extroot or if a non-LuCI + very bare-bones installation is sufficient for your needs.
Assuming you're going to utilize extroot, you may want to include the packages for the extroot process to make the process a bit easier. There are a few threads about this, such as this one, where there are some recipies suggested (mine is in post #12 of that thread, my last attempt was successful on 17.01.4, haven't tried with 18.06.x).
good luck!