Arduino only has one CPU, if you want to realize multi-threading on one CPU, it is finally realized by software.
Arduino is a convenient, flexible and easy to use open source electronic prototyping platform, including hardware (various types of Arduino boards) and software (Arduino IDE).
It is built on the open source simple I/O interface version, and has a Processing/Wiring development environment similar to Java and C language.
Look at the actual situation. If there is only one and a half input I/O, you can use a Nano board instead, which has two more analog inputs than UNO.
Therefore, if possible, I personally think that mega2560 is better than UNO+ extended I/O.