when yo say “at the moment of transmission the frequency changes to 500 Hz” is that 500hz on the WSJT-x waterfall or somewhere else?
above the waterfall you have two markers a RED upside down bracket (this is your TX frequency on the waterfall) and a GREEN right way up bracket (this is your RX frequency on the water fall
you can set your TX frequency by using “Shift + Left mouse click” and your red bracket will move to your mouse frequency,
or if you right click on a frequency you can select “set RX & TX offset” and this will set both rx & tx to same frequency
there is also a “Hold TX Frequency” checkbox which when checked will prevent your TX frequency moving when you double click a callsign in the list of recieved signal and only you RX frequency will move to the signal you want to contact, leaving your TX frequency in (hopefully) a clear slot in the waterfall
as a side note, when bands a busy try FT4, its quicker QSO time, as 7.5 second cycle instead of 15 seconds of FT8, and less popular so you can get some great DX contacts as less QRM
hope this helps
(Not Pete M0PSX who runs this site)