But, if you are only willing to vote for a party that is big enough to have real chances of winning a seat, aren’t you getting yourself into a deadlock situation here? I mean, if everyone acted like this, there would never be a change in the current party distribution. And you would never be able to give your vote to a party that better represented your values.
I would even argue (non-empirically) that you are actively hurting the efforts of such small parties. Maybe the small party would not win a seat this time, but it would gain a vote and maybe a few percents. This in turn would be a signal for other voters to consider this party during the next election.
Your statement that the cost of voting is too high compared to the impact might have a very frustrating effect on the very idealists that currently try make rational/secular politics happen (kind of ironic, since you are one of them yourself). I think your vote would mean much needed psychological support to carry on with their work. And because they don’t have so many votes yet, your vote would even weigh in disproportionately high.
I think that the dilemma you describe in your original post (which makes it impossible to give your current vote to one of the major parties) would be the perfect scenario to support the small party of your choice.
You could even write an article about why everyone should do the same, thereby further increasing your impact.