Registers & peripherals look the same, it also appears to be ISP-programmable. Pin number and footprint will not be an issue. I am mainly interested in Wire and SPI libraries, and a few of the avr_enable_* / avr_disable-* functions; no bootloader, not much else, flashing will be done via avrdude.
Does it sound easy enough to use these Atmega165 MCUs with my already existing ArduinoIDE code ? Same question for all other Atmega product lines not already supported by third party hardware packages such as MightyCore
Of course you are, great news!
I don't suppose Wire library poses any special problems either.
No such MCUs on my hands yet, but I shall likely resort to them as most other ATmegas are nowhere to be found.