That's a viable voting strategy, if you're voting against one specific candidate. But how often does that happen, where a voter truly has no preference between two candidates? But that's hardly ever the case, and STAR voting strongly discourages running that kind of capaigning. Candidates want to build coalitions and find common ground, but also differentiate themselves without coming across as negative.

https://www.equal.vote/star_vs_rcv

Nah, doing that will actually make your ballot less useful in cases where you approve of both the candidates that made it to a runoff round, because you ranked them both the same, and thus your ballot can't count as a "vote" for either since you indicated no preference.

For single winner this is will probably be a rare issue but for multiwinner, which is what I want, that could end up biting you as the margins close in for the last seat

Ranked choice voting with room for blank spaces?

Haven't done the math on how that would behave exactly, but you could effectively weight the rank of one group of candidates higher and one group lower while your total contribution is still the same, which has a similar effect as you mentioned with STAR but still forces a relative ranking to show preferences in between candidates you approve of and those you disapprove of.