It sounds like the OP has made their decision, and wants to avoid "the big three", so this is likely a moot point. But ...
Dependencies are always a trade-off. Be they code libraries/modules, programming languages, network protocols, services, etc, any dependency has the benefit of giving you something you'd otherwise have to implement yourself with the downside of then being tied to their schedule, quality, license, pricing, lock-in, etc.
In your RDS example, a similar problem might be a self-hosted database that has an urgent security update. If you're using a cloud provider, that's just taken care of without you even seeing any downtime. When self-hosted, that's a software upgrade, with all the attendant concerns about backwards compatibility, making sure you coordinate the replica upgrades, etc, etc. In both cases, ignoring the issue has costs (one financial, the other security).
In implementing a system, it's your task to balance these tradeoffs. In an investment-funded startup, often the evaluation of the costs and benefits is a little different to what it might be for a bootstrapped startup or an ongoing business. With a limited runway and resources, leveraging the work of others can be a lot more attractive.
Free plans do tend to make expanding your set of dependencies very easy, and cloud-based infrastructure is certainly expensive when compared to the direct costs of self-hosting. But it's there now, and will be there with no ongoing effort on your part. Sometimes, that's worth the cost for the time to market.
All that said: this isn't an either/or. Digital Ocean has a bunch of services that you might choose to use too, and they're not "the big three".