I was 3 when TVH came out, but I seem to remember there being chatter of it being the last movie. Was that actually considered?
ST:TMP was assumed to be "the only movie", because it had morphed from an intended sequel TV series and then, after lacklustre reviews, it was doubted there would be sequels, at least not at the cinemas.
ST II was then assumed to be "the last" movie. Until Nimoy expressed interest in directing a ST film.
ST III was assumed to be "one last movie" by most of the cast, even though their contracts had an option for more. The model makers, though, planned that, if the movie series continued, the heavy and hard-to-film ship model would be superceded by the smaller, lighter Excelsior model. So there was some planning ahead to possible future projects.
And yes, ST IV was also assumed that the cast had "one last movie" in them - until a few months before the premiere where it was realised the franchise was now stroong enough to support the planning of a new TV series.
And ST V, ST VI and "Generations" were all, at one time or other, "one last movie to go out with a bang".