Building deep-tech (say, new kind of radio link) involves years of R&D, specialized manufacturing, and long regulatory or testing cycles. It's uncommon in app business though.
1. Strong hierarchical structure is prevalent in that team, so, you'll have to bide your time (amicably), if you've to been taken seriously.
2. Expectation from you is to raise PR until you 'know' how it's done. By showing up from bigtech and by suggesting solutions, you are basically telling them you've seen it done better, and that's a bitter pill to swallow.
3. The startup has been around for 10 years, and tech changes fast; things might have been built rapidly and to a paying customer's corner case, and now the product looks terrible from an external observer's perspective.
4. Even bigtech products evolve into terrible things and nobody has time to go fix on a haunch from someone, however insightful it is. If you claim to have tech skills, why not come up with metrics to prove your point and how much money it can save or bring in.
5. You can't convince people from your pedigree, but from your insights AND backing it with data (latency, resource usage ==> revenue).
6. Startups don't have money laying around so if features don't have $$$ printed on them, they get canned.
recently i was helping a a friend purchase a custom domain, and signup to a email service; even though i use fastmail, i was surprised to find proton was like 3eur per month (fastmail is 5eur). when my renewal expires, i will probably switch to proton - for 10eur the features are really good;
If money is not an issue, would you consider an open-source project to contribute to, which could look attractive when the hiring begins?
ignore all FUD on LLM; the professional products would need developers now and in the future.
don't want to sound patronizing, but if you could use this 'real-world' scenario (as tough as it is) to build robust coping skills, it might benefit you personally.