It's still worth it to register for one of the major parties to vote in their primary and push them towards your actual politics. For example, I wouldn't consider myself "a Democrat", but I am registered to the party and I vote as progressive as I can in primaries.
Not all states work the same. In Ohio I can just show up and tell them which one I want to vote in each time. I always vote in the Democrat or Republican primary, I get a voice without committing to one or the other.
From what I've read, the two times Trump won, many Democrats felt that they were denied this choice, which left them disillusioned, and they didn't vote. I don't think that's the main reason for Trump's victory, but what you touched on was definitely a factor in the Democrats' loss.
I wonder how many times we need this to happen for people to learn that letting others make your choices for you will often lead to your worst possible option.
In a lot of places, you pretty much have to register for a party to have any say in the local elections. Where I live if there is a primary for an office it is guaranteed whoever wins the Republican primary is going to win the office, so if you want to have a vote in that election you have to register Republican.
I think everyone should get a vote in every primary. If there is a Republican and Democratic primary then you should get to cast a vote for a Republican candidate and a Democrat candidate. I think this would result in a better selection of nominees for offices.
And some of us who are that way understand that in FPTP there can only be a winner from one of the major parties and we are choosing who we want to fight to push for changes.
I always vote for who I perceive as the lesser evil of the two. This year is no different. I'm not excited about what either candidate wants to fight for. I will oppose whoever is elected on multiple fronts.