I think Windows Server 2012 R2 supports VMs in the 5.x configuration version with Gen1 fixed-size VHDs whereas by default your Win10 HV manager will probably be using 9.x with Gen2 dynamic VHDXs. If it's not "seeing" them at all, check to see if it's simply because they are VHDX as "not seeing" in Windows is generally a matter of file extension differences.

Cost-Benefit Analysis

Yes it can be done, but you won't be able to go back and forth. AFAIK there's no way to do it directly. You have to make a sacrifice.

You have three ways to move them, none of them can be done without changing everything in some way. In practice, generally Linux distros can be rebuilt and individual files copied over as a quick solution. Your next option would be to remake the VMs in WS 2012 with downgraded VHDs, then cloning the file systems over using a disk cloning application. Finally, if you want to do what I think you really want to do, Hyper-V Server 2019 is free, you can host machines to your heart's content and move VMs back and forth at will.

Rebuilding and moving configs will be the fastest and easiest:

If you want to keep your Windows Server for all its other resources (and GUI!) then this is your best option.

Rebuilding Linux distros for specific purposes is something that you can prepare scripts for and this is a great chance to practice making them.

Create the new machines from scratch and move over any files that you realize are missing once they're installed. If you haven't learned about it yet, WinSCP makes this a breeze. You can enjoy a handy Windows interface as you search your Linux installs and copy over any files.

Downgrade your VHDs and insert into new (older-versioned) VMs

1. You create the new VMs with hard drives that are whatever size you want them to end up as on WS 2012.
2. You move those hard drives over to your Win 10 machine.
3. In your Win10 VM Manager, create a new VM without a HD that boots from from a disk cloning ISO (like Gparted Live). Don't boot yet.

If it were Windows only, we could use diskpart or dism but because those are going to be EXT file systems those won't work. If GParted gives you trouble, you could also try an imaging program like Win32 Disk Imager but you might have issues going from GPT to MBR that it can't compensate for as easily. Aomei Disk Cloner is another freeware candidate.

4. In your Win10 VM Manager, attach the old and new matching hard drives to that same VM you just made. Now boot it.
5. In the Gparted GUI, clone the original (gen2 VHDX?) hard drive to new (downgraded/gen1 VHD) hard drive.
6. Move the cloned gen1 VHD hard drive back to your WS 2012 machine, make sure that it is set as your primary/only hard drive, make sure that you disabled safe boot (not sure if it qualifies for VMs that old) cross your fingers and pray.

Here are two guides that have similar processes for other people's versions on how to do similar things:

https://www.magrin.one/hyper-v-gen2-to-gen1-vm-conversion.html

https://4sysops.com/archives/how-to-p2v-windows-server-2012-r2-with-uefi-and-a-gpt-disk/

Move them back and forth without CredSSP issues by upgrading your server for free:

This is the best permanent solution to keep up-to-date with updates and support and cross-compatibility, but will take a while until you figure everything out. If you want to do other things with your Windows Server, skip ahead to the next section.

If all you want is a server to host VMs, you can get the latest and greatest for free from Microsoft's official site: https://www.microsoft.com/en-us/evalcenter/evaluate-hyper-v-server-2019

You'll get a free unlimited license for Windows Hyper-V Server 2019 by just providing some personal information. Although it has no GUI and just a command prompt in a window, once you get used to it (and used to using notepad as your impromptu "GUI"...and get the CredSSP thing figured out) it should be the fastest way to run your dedicated VM server.

Make sure you remove your laptop from the domain first!

Anyway, whatever you do, it should be a lot of fun!

A side note:

在域外使用 CredSSP 跳环访问另一台计算机的 Hyper-V 管理器是一种痛苦。仅出于此目的将自己添加到域中会向您的计算机添加一堆域垃圾，如果您在进行实验时不小心，可能会将您锁定。此外，当使用 Hyper-V 管理器控制远程计算机的 Hypervisor 时，缺少足够多的选项，因此不值得。我建议（只要您使用 WS 2012）从现在开始，您只需通过删除桌面 (RDP) 连接到服务器并以这种方式访问​​其 GUI。如果您还没有尝试过 RDP，那么它非常适合。