Should I worry about vendor lock-in?

Less than you think. ACC is just Magento 2 + AWS + Fastly + New Relic. Your codebase is portable, same composer.json works on a self-hosted server. The only ACC-specific bits are .magento.app.yaml + .magento.env.yaml + ECE-Tools dependency, all of which are open-source. Realistic exit time: 4-6 weeks to migrate from ACC back to self-hosted, vs 8-12 weeks the other way (because you’d need to set up the hosting from scratch). So lock-in is real but recoverable, much less sticky than e.g. Shopify.