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> The SWD clock frequency is 16 MHz.

That's ridiculous, turn it down. You don't need anything beyond 1-2MHz for programming, there is nothing during programming that _needs_ a high baud rate, so it can get turned down significantly. 

1MHz might be a good compromise between speed and reliability. Even for some >100kib project it gets erased & programmed more or less instantly anyway. Connecting the cables take far longer.

Debugging close to real-time is another matter, but then you would always keep cables short. And there's just so many scenarios where you need to debug close to real-time with SWD. If your MCU doesn't have instruction trace or you aren't debugging by watching trace, then 16MHz is nonsense for debugging purposes too.
  
> The SWDIO signal, however, is bidirectional

So _don't_ use 74LCV125. Use something like 74LVC245. By using a unidirectional buffer IC instead of a bidirectional, you are only creating artificial problems needlessly. Either part is cheap so part price is not a reason. LVC245 works down to 1.65V so it should be no problem even with 1.8V supply. 

Ground /OE, keep things enabled at all times.

In general, I strongly recommend the [KISS principle](https://en.wikipedia.org/wiki/KISS_principle) for designing hardware and software both.