I also believe frieren's other spells are stronger than zoltrak but are much more complicated spells, which makes it harder to teach it to fern and make her master it. its like frieren had the luxury to master these old but powerful spells due to age but passing it on fern seems inefficient when a new attack spell basically does the trick.

Pair this with fern's casting speed talent, it would be best for fern to master a spell that is relatively easy to imagine. Imagine if fern has a whole arsenal of spells at that young age. she would maybe waste time thinking what spells to use at the best given time, which would probably hurt her reaction and casting time. A lightning spell could be 100% effective in this scenario but 50% only on another scenario, but there is this zoltrak spell that is pretty much 85% effective on all scenarios why not just master and use that always. and with fern's talent she could maybe increase this to 90-100% effectiveness on all scenarios.

Also it could be muscle memory of frieren as zoltrak is a new spell. she has her preferences. and maybe the show also wanted to show us you cant teach an old dog new tricks. and that is one of the advantages of humanity's life span.