The downsides of not going straight to proxmox are all pretty much permutations of missing out on features or having to deal with a migration later on down the line when you do switch to it.

Those features are almost universally stuff you might decide to not use or to use in a particular way, so it’s easy to say “pump your brakes and get your feet underneath yourself first” before handing you a tool that can be configured (with the help of Reddit, stackexchange and llms) in infinite wrong ways.

Kind of like suggesting someone learn how to make a simple miter joint before handing them the universally loved and used cordless oscillating multi tool. The tool is really powerful, but the skills and foresight doing even just one miter joint will give you let you make better choices about how to use the oscillating multitool when you have it.

Migration from bare metal to literally anything else is incredibly well documented and not a big deal.

Often times for some of the stuff you said you’d be running there are guides for migrating that particular package from metal to containers, vms, or to proxmox itself.

I want to make it clear that everything you learn from bare metal Debian would transfer over and compliment learning skills directly with the proxmox package because proxmox runs on top of Debian and Debian would likely be the os your vms or containers are made from.

You don’t need to throw yourself in the deep end to learn how to swim.

E: there is the extremely rare possibility that you will have some crash or security problem due to lack of containers/vms. I say extremely rare and I mean extremely rare. My personal server which was bare metal for twenty years just recently had its first one and it was actually related to a problem with containerization as opposed to lack of it. Your mileage may vary but for home users who don’t have public IPs and services getting pounded on 24/7 it wasn’t even something I thought about.

Another pov/idea perhaps - you can still run proxmox & only one virtual machine (eg standard Debian, no performance loss), but perhaps gain backups/copies if you plan to eventually add a cheap second machine (location?) instead of local redundancy.

Also with proxmox you can copy you virtual machines/containers & experiment easily.