Linux

The most painless way to dual-boot is to install something that's not Windows alongside something else that's not Windows.

Get a second pc and a kvm switch

If by "these days" you mean a motherboard that supports UEFI, then it honestly doesn't matter anymore. Your board controls the boot order, and there isn't an MBR for Windows to mess with anymore. Just plan out your partitioning careful before hand, and if you plan on using a lot of files in both OS's, make a plain storage partition that is easily mounted under Windows (NTFS does not count).

Honestly, aside from a very scant number of apps or games, there isn't a real need to dual boot anymore. If you can't run something under Proton or Wine, having a Windows VM will get the job done.

As for the second question: Windows 11 IoT LTSC has yet to be mentioned here - the only things that can stop you from using it are legality and convenience.

I'm not sure if W10 has an IoT LTSC version, but W10 LTSC does exist.

If you insist on dual-boot: two separate physical drives.

Otherwise: use linux with a windows vm.

You can (at least the last time I ran an install) get both 10 and 11 installed without a Microsoft account, 11 just requires this process to do it. If you have an old ISO of 11 around it should allow a local account if you don't connect to the internet, but they apparently patched that out now.

Swappable hard drives

I have a ThinkPad with easy access to the hard drive. It's one screw, remove a small panel and slide out the hard drive, slide in a new hard drive and reinstall the panel and screw. It all takes about a minute.

I have a drive for my Linux setup and another for windows.

I gave up setting up dual boot setups because I'm not as skilled or capable and I've lost entire setups in the past due to updates and changes and it was constantly frustrating for me. So I figured that just swapping hard drives was the easiest for me. No configuration, no changes and neither OS can interfere with one another.

I use my Linux as my daily driver for everything and windows when I need something from windows. I only ever use windows maybe once a month or once every second month. I spend more time regularly updating windows than in actually using it.

Having another PC with minimum requirements only for Windows?

I put windows 11 live on a £20 USB drive, and it hasn't messed with my Linux install at all

Getting a second drive just for windows I think is a good approach. If you were to do so, it's important that you remove all other drives while installing windows, otherwise the Windows installer will put its boot files into whatever existing EFI partition it finds.

Then using something like https://github.com/Raphire/Win11Debloat you should be good to go with a relatively clean setup.

To have a local account, I use Rufus to setup the usb installer in a way that it automatically creates the local account, and it can also disable the secure boot and tpm requirements from the installer if you want. Though I think rufus is a windows program only. I know there's the "OOBE" approach for the local account, but I haven't done that before. That could be an option too

Others have answered your dual-booting question pretty well. However, along the lines of "minimal" Windows, it's not generally recommended to fuck with the system as that can break things. There are scripts that can strip a lot of the problems, though. I can't remember any off the top of my head.

As for not requiring an account, I have old ISOs of Win11 and Win10 where the unplugging my ethernet cable trick gets me around signing into a Microsoft account. Not sure if it works on the ISO you get from Microsoft now, however. And if you have built-in WiFi, I think there's a way to disable it in the command prompt before you install.

Edit: Win10 is going to hit EOL in the near future. I am going to use it until then. It's got a lot fewer concerns (for me) than Win11, unless Microsoft keeps filtering Win11 shit into it.

Is there a reason you need a dual book instance instead of a VM or even WINE?

Unless you need direct access to hardware and if you have enough RAM, you can probably avoid dual booting altogether.

Two physical drives. Install windows first in one, then Linux on the other. If you don't do this order windows boot manager will take over and you'll have to boot Linux from bios.

My first line of investigation here would be virtualization. It will solve the "don't mess with my Linux install" problem and will let you use the windows apps you need at the same time as the Linux apps you normally use. Also VMs have all their other useful features like snapshots and portability.

I did this in the distant past and it was quite convenient having the VM instead of a dual boot.