[kelnos]

Since the Pi4 can boot from USB now (might still be in beta, not sure if it's fully released), I would get a USB3 to M.2 adapter, buy a decent M.2 drive, and use that. It'll take up more space than the SD card, obviously, but not that much more.

[Chernobog]

You can even get SSD based USB sticks like the Sandisk Extreme Pro 3.1: https://www.amazon.com/dp/B01N7QDO7M

I bought one to run Windows on my Retina MacBook Pro. I only need Windows for gaming when visiting friends, and it works flawlessly for that purpose.