What about using once() on the geotrace calc, along with adding a condition to the relevant column that the geotrace isn't calculated until you have 2 repeat entries, using count()?
I don't know if this will work or not, but I did use the relevant/count approach when geoshape & geoshape were first released to keep from having to hit the refresh button on the geoshape once 3 or more repeats were added. I'm assuming that using once(sum(${repeat_location}) for the geotrace calc would work, but haven't tried it.
edit: After re-reading, hopefully I'm not misunderstanding the question too. I'm assuming that you're just wanting to calculate a 2-vertex line out of any random two geopoints in a repeat, and that it only needs to happen once, and you don't care which 2 points. Now that I noticed you're creating a tool for surveyors, I'm thinking I misunderstood the workflow. Is this just a tool for them to input coordinates and calculate a projected bearing and distance as-needed or something? I guess a different approach if that is the case could be to create nested repeats, where the the geotrace is contained within a repeat and max-2 geopoints are contained in a nested repeat. I assume this would allow the surveyors just to add + a new repeat (the geotrace) each time they need to do a new calc, instead of changing the existing points and refreshing the existing geotrace.
Apologies if I'm way off the mark on one or both of these comments...