This is not a junior consideration and it's not unrealistic that firebase ends in say, the next 60 days which in terms of porting your entire application is essentially today. You're right to weigh this as a concern in evaluating approaches and everyone should, regardless of experience.

All PaaS providers work hard to couple you to their platform not (necessarily) for nefarious reasons, but because that's how abstractions work. You need to be continuously vigilant and aware of when and how you are tied, do a cost/benefit analysis and have risk mitigations for things like your original thought, so good for you.