In your first example, in addition to selecting the Pro Mini in the IDE, did you also select the 8MHz version? It just seems to me that if you were able to upload a sketch via the FTDI once, there's no reason why you couldn't do it again unless your IDE settings are different, or there's a bad connection. Nothing would have changed the bootloader.
Yes, I'm familiar with this. I have plenty of pro-mini boards, both 8MHz and 16MHz. They use different baud rates depending on both the MHz and the type of bootloader installed on them. At this point I have MCUDdude "MiniCore" on all of them. There have been recent updates to both MiniCore and the Arduino IDE itself, so I'm in the process of confirming I have the most recent updates installed. I do need to install the latest Arduino IDE package (1.8.19) because I'm still using 1.8.9, which has worked perfectly until I got these new clone pro-mini boards. The PC is Windows 10.
What I find strange about these pro-mini clones is what happens immediately after using ICSP to erase flash and install a bootloader (this leaves low flash filled with 0xFF). At this point I can always load
one sketch to the board with the bootloader. The sketch works fine, but afterwards the board fails to upload another sketch through its bootloader. Curiously the Arduino IDE “serial monitor” connection always works. Only failure is the bootloader stops working. I've been using the same FTDI USB-to-serial interface for everything. Perhaps it’s “reset out” pin has failed? I have several other USB-to-serial devices which I will try next. Sooner or later I will identify the "weak link."
One interesting discovery from today is that some of my 8MHz pro-mini boards (all Chinese clones) function perfectly when I use their 8MHz internal oscillator, but they fail to connect to the Arduino IDE when using their 8MHz resonator. I plan to activate the oscillator-out pin to measure actual clock frequency with my frequency counter. I'll use my ICSP programmer to change the clock fuses. I suspect the Chinese clone makers use the very cheapest resonators they can get. Most of my “Uno size” boards have actual crystals. But small boards (nano, pro-mini) have tiny resonators which may not be as accurate in frequency as crystals. I do have several 16MHz nano clones which always work perfectly even though their 16MHz comes from a resonator. I also have several SparkFun RedBoards (same form factor as UNO) with 16MHz resonators. They are totally reliable at all times, but then they aren't made out of the world's cheapest parts. Conclusion: Resonators aren't always "worse than crystals."