Wsjt-x emits PTT via Rig_Control, then, with {user adjustable delay, default 200 ms}, baseband starts.
I have homebrew Linear Amp with bias and antenna switching.
The FET driver has asymmetrical +/- supply rails so that zero cross is adjustable relative to gate threshold .
During receive, the driver (+) rail (adjustable +9 to +13V), is clamped to 1.2 V dc with Vg at -2.0 V .
The Hermes 14 SDR Txcvr has an open drain that pulls low at PTT.
Initiation of Tx sequence:
Receiver reed relays (fast) open antenna and short the receive input.
The 13.8V to 200 V drain converter is started and ramps up.
The Tx contactor is energised, to close.
The driver (+) rail clamp is released and ramps up in few ms.
By the time the RF starts, the 200V supply has already ramped up (about 150 ms)
If you sense RF instead of using Rig Control PTT, I think it should work OK.
Take care that the TX antenaa switching is before RF starts.
The loss of 200 ms or so should not affect wsjt-x decoding, and should be OK for ssb too.
CW might need some extra consideration...