Have you tried a 1/4 wave vertical with 4 radials sloping towards the ground? (all wire)? or even a vertical dipole fed halfway up? J-poles show significant coupling to their feedline and mounting arrangements and, as you discovered, are taller than they need to be for their gain.
A common thing I see is people running these part-15 (in the US) devices with upgraded antenna systems, which isn’t permitted. Either run it under amateur rules or under part 15.