Yesterday I uploaded a stable firmware build to the github releases page. Running the rp2040s at 266MHz with a qspi clock divider of 4. This means the PSRAM can be accessed at 66MHz, which with all the testing I did with DMA and where to place the statements for fetching data, means I can comfortably hit N64 PI bus speeds of 0x2040, which is a bit away from the stock speed of 0x1240 but functions well enough for most games.
Currently only EEPROM (both 4KB and 16KB) are saved/loaded from the SD card. FlashRAM support (1Mbit saves) aren't yet supported nor is SRAM. SRAM works while the the cart is powered but I'm still figuring out the best timing to send that data for save. I thought that the reset button on the N64 would give me some kind of alert so I could use that to dump saves before the user powered off. It might, but I haven't been able to figure that out yet.