Your only real bet is to rely on things that are cross-cloud by default (Kubernetes, S3 or something that can speak the S3 API). There's no real way to go multi-cloud without lock-in. There's no real way to go single-cloud without lock-in. There's no real way to go on-premesis without lock-in. Computers just kinda suck these days because of the capitalist rent-seeking powers that be that demand that everything be forged from the mines of pure suckitude.