I think I understand why you strongly disagree with the label. Those who call your license "source available" without qualifiers are, from your perspective, committing the noncentral fallacy (https://www.greaterwrong.com/posts/yCWPkLi8wJvewPbEp/the-non...). While your license is technically "source available" according to the Wikipedia definition, it is not a typical example of a "source available" license, which usually doesn't grant additional freedoms over time and doesn't become free-as-in-freedom. I don't really see a fix. The difference between "technically category X" and "typical of category X" is an eternal universal source of bitter conflict. The best advice I can give to people embroiled in one is to care less about "technically category X" if at all possible.
Besides inventing a new label and adding a qualifier like "delayed open source", one admittedly unlikely thing licenses like FSL could try is to "reclaim" "source-available". It doesn't have to mean "a megacorp lets you peek at the code if you agree to not use it for anything interesting". The typical expectations of a source-available license could shift to less onerous and less restrictive.