Try wrapping the plug with PTFE plumbers tape. It works for me with new or re-used washers and in between the threads causes progressive tightening and a super seal.
. Yellow PTFE
though they don't change the colour of the tape itself in the UK..only the roll-holder is merely plumber's tape that is petrol-safe. It will not degrade as many plumber's tape reputedly will when used with petrol, oil etc. Costs the same..you merely have to search for it. Tape is a surer bet than washers, though washers are more quaint.
Of course, you can use both at the same time for redundancy.I used to use copper washers and still like them. One must buy a few at time as they are size sensitive, (unlike our tape). They CANNOT be re-used. Like you, I have found plumber's tape more reliable and it handles problems of knackered threads on both sides and owners who torque to eye-popping. You merely have to use it properly. I'll search out a decent youtube video now of instructions, though wrapping threads a little is certainly not rocket science and a roll of the right tape costs around 1-2 pounds/dollars.
https://www.youtube.com/shorts/efTqIIAfPhILorne (who loves redundancy)
