This is one of the things (along with repair) that finally motivated me to learn electronics. You could probably build your own Everdrive 64 for less than $20! (Plus hundreds of dollars for an oscilloscope and other equipment but you can keep that for the next project)
# If you have more than 2MB flash, you need to change the flash size by adding -DFLASH_SIZE_MB={one of 2,4,8,16} here.
2MB might be a minimum, as the standard bootcode needs 1MB + 4KB. See my boot_stub or modified 6102 bootcode for ways to bypass the 1MB checksum: https://hcs64.com/n64info.html
I think it's actually a limitation of the Raspberry Pi Pico only having 2MB of flash space while some other RP2040 boards can support up to that 16MB. Reading SD cards with the PIO might be a bit much so the project README mentions the 'next-gen' version with added RAM
Maybe worth it?