I still don't totally understand why the whole V'ger thing couldn't in some way be connected the Borg, but that's another long back and forth thread that's been over-analyzed to death.
There are at least five very straightforward reasons why not.
The first is that the Borg hadn't been written yet when TMP was created, so the writers of TMP had no intention of involving the Borg, by default.
Second, when the Borg were created, there was no effort to connect their origin with V'GER. Neither the origin nor the fate of V'GER has been mentioned in on-screen canon outside of TMP.
Third, if Spock is right that the machine planet is represented in V'GER's memory, then V'GER digitized the machines that modified Voyager 6!
Oops! Except that they were machines and probably didn't feel any emotion. Ergo, if it was Borg or a Borg precursor all on one planet, then V'GER would have gotten rid of the Borg.
Fourth, Spock called them machines and not cyborgs.
Fifth, the construction of V'GER depended upon machines being unable to fully decipher the Voyager 6
probe. But full-blown Borg, having the experience of an entire collective of assimilated carbon-based life forms to draw upon, should not have had that fundamental handicap.
There are just too many things that don't jibe and which would require tweaking and qualification, for it to be a natural fit.
On the question of whether TMP was a good enough start, I was disappointed with TMP when it first came out, and I still am. However, when stacked up against the other Star Trek
films, I rank it near the top. This tells me not only that making good Star Trek
movies must be a lot harder than I want it to be, but also that probably my expectations at the time were way too high. And that's not to say that TMP is objectively awful, because it isn't. It's not awful, but it has a lot of problems. It also has a lot of good things going for it.