Hi,
I have experience for 15+ years in electronics/embedded development. I have several years of advanced AVR/arduino development, including library development from scratch. Practically all Arduino AVR boards used, like: 2009, 2010, UNO, MEGA, NANO, MICRO, MINI, DigiSpark (ATTiny), etc. Several shields used like Ethernet (W5100, ENC28J60, etc.) with various IP protocols implementation, various graphical TFTs with and without touchscreen and various BUS types (parallel, SPI, I2C, etc.), all kinds of sensors and buses, serial and parallel memories, etc.
I recomend for you project using the arduino framework for simplicity to replicate and program the MCU on your side once completed on my side. If you require, I can also use AVR Studio / Atmel Studio instead.
I don't have any ATTiny441/ATTiny448, but in case you award me the project, I can get some that should arrive in less than a week. Meanwhile I can work on the firmware and when the MCUs arrive, I'll test it out and debug if needed.
For the firmware requirement itself, converting a "9O2" to "8O2" UART communication, I don't think I'll have a problem with that, as I have already implemented similar features in several projects before. From what I got in your description, the translation only needs to be from one UART at "9O2" to the other at "8O2" (discarding 9th bit), but I can also implement bidirectional translation if needed.
If you are interested in my bid, feel free to contact me.
Regards,
Carlos.