Veeam is a good start because they already support a bunch of hypervisors and are working on more. But for example, Veeam can't yet do replication with Proxmox, but it can with VMware and Hyper-V.

For hardware, I'd avoid going in on hypervisor platforms that need you to buy their specific hardware. Your standard Dell, HP, Lenovo servers can run almost anything, but if you buy a hyperconverged system you are going to get yourself locked in.

A big lesson I learned is: Make sure to divide up your storage pools enough! There's no easy way to gradually migrate if your storage array is one big VMware VMFS file system.