Install Windows 11 on unsupported hardware or still using Windows 10 with some security means.
Running Windows 11 on old hardware with an unsupported CPU and (possibly) Secure Boot and TPM disabled is no worse than running Windows 10 on the same hardware with the same security settings in the BIOS, until later this year. For all I know, you're already running Windows 10 with Secure Boot and TPM2.0 enabled.
The bit you need to be concerned about is whether or not the version of Windows you're running is patched and up to date. Different versions of Windows 10 have different end-of-support dates up to 2032. See below:
Standard Windows 10 Home and Professional will be patched until 14 October 2025.
https://learn.microsoft.com/en-us/lifecycle/products/windows-10-home-and-pro
Windows 10 LTSC 2021 until 12 Jan 2027.
https://learn.microsoft.com/en-us/lifecycle/products/windows-10-enterprise-ltsc-2021
Windows 10 LTSC 2019 patched until 9 Jan 2029.
https://learn.microsoft.com/en-us/lifecycle/products/windows-10-enterprise-ltsc-2019
Windows 10 IoT Enterprise LTSC 2021 until 13 Jan 2032.
https://learn.microsoft.com/en-us/lifecycle/products/windows-10-iot-enterprise-ltsc-2021
Windows 11 Home and Professional - various dates for 21H2 to 24H2.
https://learn.microsoft.com/en-us/lifecycle/products/windows-11-home-and-pro
VPN
A Virtual Private Network simply hides your IP address (assigned by your Internet Service Provider) from anyone who cares to check. With a VPN, you can make it seem you're located in a different country (to a casual observer). A VPN does not confer any greater degree of security to your connection. You're just as much at risk using a VPN as your ISP on its own.
It's up to you whether or not you install Windows 11 before 14 October 2025. Some poeple are still running Windows 8, 7, Vista, XP, but if they have any sense, they won't connect to the world wide web.
If you need Windows 10 after 14 October for programs which are "unhappy" in Windows 11 and you install Windows 11 Professional on your PC, you could enable Hyper-V and run Windows 10 as a Virtual Machine inside Windows 11. It might be wise to disable internet access to the VM, but that's up to you.
https://www.solveyourtech.com/how-to-create-a-virtual-machine-in-windows-11-a-step-by-step-guide/
It gets more tricky if you need to "pass through" your GPU to the VM, but it is possible.
https://www.nakivo.com/blog/hyper-v-gpu-passthrough/
As for extra security, you could build a pfSense firewall in an old PC, but it won't stop you from downloading a virus.
https://www.pfsense.org/