Hhmm, I don't think a display is a stretch for Anduril on a AVR ATTiny. There was one, maybe two threads here on incorporating a display with a custom driver and think they used a PIC, but still an 8 bit MCU. Even with 16K Bytes, the build I've done for the 1616 is still like 9200 bytes, so 44% code space still available.
Here: https://budgetlightforum.com/t/-/42991 and https://budgetlightforum.com/t/-/61255
Some Background:
https://budgetlightforum.com/t/-/43444
https://budgetlightforum.com/t/-/63478
https://budgetlightforum.com/t/-/26036 (goes back to 2014)
I found he mentions: PIC16F1575, but not sure that was the only PIC. tterev3 was way ahead of his time here on BLF, and many of us didn't really know what to do with this tech.
I don't know, the more I read about these mods, there was little we could do but be amazed. Too difficult for most of us to do those type of mods, didn't hook up with a manufacturer or even someone who could make the parts that I know of. He did post source code at one point but think it was written in ASM, if so, difficult to work with.
I was very interested in the MELD RGB stuff at the time, but was scared off by his abilities with micro wiring - didn't think I could do it. tterev3 is super talented in many ways.
For the I/O to a OLED display, I believe the USART/SPI/I2C would have it covered.
His latest EC01 mod uses the PIC18F26K40.