First, I want to acknowledge that @pcort handed me my ass on three different TT hill climbs last season, so his advice is worth heeding.
If you are looking at 30 minutes, you should be able to ride at threshold for all of it and still be able to light a match toward the end for the kick to the line. You already have an n=1 example of feeling you had more to give. Don’t let yourself have that feeling on race day.
This isn’t a long race - I say go to the highest level of discomfort you can tolerate and hang on for as long as you can, which will be the whole race.
A lot of racing on climbs is commitment to suffering more. Embrace it.