Not entirely sure if this applies to the 2019 model, since I haven't been able to test on one - but I know for the 2015s, using a different model LCD will be seen as an external monitor - which should make no difference in performance or resolution - unless you had a mid 2015 machine (would display 4k on 27").
But with a different model LCD on 2019, you might not be able to get 5k resolution on any external monitor you connect through TB3. Only a theory, since again - I haven't been able to test this on a 2019.