Why did you look for other sources of APIs the Bitcoin core or full node itself has RPC API you don't need to look for other sources unless you don't want to run a full node.
FYI, even if OP or someone else doesn't want to run their own full node, they could use service (such as getnodes.io) which offer Bitcoin Core API.
Those online providers of RPC APIs have a serious limitation. Because they only allow so many (lat's say X) amount of calls a day, it means you cannot use their APIs for building a transaction database which would otherwise let you work offline, because that requires literally millions of RPC calls which are way above what any provider is willing to allow.