what specific liquidity requirements you have? just some inbound and outbound for initial testing?

the most reckless possible option would be to use demo lnbits server at https://legend.lnbits.com/ for prototyping but this is highly inadvisable. But it gives you very clear path to move the project to your own lnbits instance later on.

zebedee has their custodial platform for developers - https://zebedee.io/developers/dashboard that gives you an api. or river's RLS -> https://rls.dev/. locks you into their system where you will spend probably nontrivial amount of time if you later decide to migrate away due to different api's and wanting to abandon a custodial solution

you could go a bit deeper with breez_sdk where you can easily spin up a node through api and have lsp support so inbound is taken care of - self custodial via greenlight