I personally prefer ROs that are directly involved in the plot somehow. This usually involves ‘party members’ but even in that case there are sometimes characters whose role can just be “romancable archer that you can talk” to without them impacting the plot.
For example in Dragon age: Orgins Morrigan and Alistair are both party members who are integral to the plot, whereas Zevran and Leliana are optional. You can interact with all of them equally (if they’re in your party) but if you dont recuit zevran for example, you don’t lose out on much except his specific side quest. So even though his romance is (imo) very well written it doesn’t feel as impactful or as important as romancing one of the two ‘main’ characters.
To use another Dragon age game as an example, in inquisition i really felt that none of the characters ( except solas and maybe dorian) were integral to the plot, so their romances all felt hollow despite them being party members with side plots and lots of interaction. In the same game there are the two non-party member romances, who i feel have a little bit more plot relevence than the actal party members. Sure, Cullen and Josephine(
) could have been replaced by another character, but I liked that they were written to have agency and drive the plot of their own accord, rather than just being soldiers for the MC to arrange in battle.
TLDR. To me its less important that the character is ‘in the party’ and more that i feel all characters should be primarily there to drive the plot before being there to be romance options.