Can RX pin(s) be used as TX instead? Title question. I do not need any receive communication pins, only transmission.

Actually, I need as many transmission pins as possible on any Arduino besides a Mega2560 (to re/write the EEPROM of slave devices). A Mega2560 has 23 TX/RX pin pairs.
Maybe I'm misunderstanding serial communication. And maybe some sort of multiplexing deal can work like One-Wire temperature sensors...such as softwareSerials[x].write()...

SoftwareSerial serialOne(2, 3); // Software Serial ONE
SoftwareSerial serialTwo(8, 9); // Software Serial TWO
                     // RX, TX


Is there a way to make all the pins using SoftwareSerial TX? Or do half need to be RX?


&quot;Just take a better microcontroller, and that is all.&quot; - user1829200

What off-the-shelf microcontroller is geared for serial communication?
One way to communicate Arduino to Arduino device is UART (and Software Serial for additional pins). For the ESP32, it seems this allows 28 TX pins using Majenko's method (in their answer). In other words, slave 28 devices.
After further research, isn't I2C the clear route to go?


With more slave devices the less times per second to talk to them all
The application needs to have all the slave devices execute commands synchronously:

At least, the slave devices need to start functioning at the same time.





How many slave Uno's/Nano's can be connected to an ESP32?


Each device on an i2c network has a 7-bit address, so a single network theoretically supports up to 128 slave devices. In practice though, the limit is much lower. i2c slave chips often support only 8 different bus addresses, no more than 8 of that chip can be attached to the same i2c network.

https://mindbleach.com/words/2012/10/23/i2c-and-device-limits
Unclear answer:

https://forum.arduino.cc/index.php?topic=159860.0
https://forum.arduino.cc/index.php?topic=367961.0