Definitely do a couple of test joints as mentioned, but assuming accuracy all round, 19mm rebate for an 18mm board will end up feeling sloppy/being weaker across the whole structure
^ This ^
Provided you’ve routed the rebates straight the only issue will be in the shelves thickness which could be a little bit patchy. Easier to sort that out on as you find it.
Definitely do a couple of test joints as mentioned, but assuming accuracy all round, 19mm rebate for an 18mm board will end up feeling sloppy/being weaker across the whole structure