If I’m reading this correctly, there’s only 5 pins on these boards that are truly unencumbered. What I mean is that during reboot or startup, there aren’t any internal functions that requires these pins to be set low or high at start in order for the ESP to function properly. I’d bet that there are work arounds, but my Arduino skill level is still basic, but the ESP takes me to a whole new level of newb. That said, for my comfort zone, I’m seeing 5 pins that are truly usable for whatever, so 4 to drive the LED side of the relay module.
I think I mis-spoke when relating to opto-isolation as it is my belief that there is no other way when using an ESP due to pin limits at 3v3. I could be wrong about that too though.
https://community.blynk.cc/t/esp8266-gpio-pins-info-restrictions-and-features-8/22872