The other reason they bleed is because, even though they do use different frequencies, they are basically “in between” the analog channels we use. Along with that, and also transmitting like other poster stated, makes it easy for them to bleed over.
I agree with other posters suggestions, couple more things, looks like you’re in the UK, and even though they should be on 25mW, you could ask them what power they’re using, since they go to 700mW, and with an easy hack 1200mW. Also, make sure you’re using RHCP antennas, since DJI uses LHCP. (This doesn’t help as much as between 2 opposite direction analog systems, but it helps a little)
If you have the money I’d pull he trigger on the DJI set up, I just got mine literally yesterday, but we’re having an ice storm here in the Midwest U.S. I love Fatshark and have many of their products, and I really wanted to dislike these DJI goggles, but after all the reviews from top FPV pilots and then trying my friends, I was just too impressed. I don’t think FS is even close to having a system that good. The Frostbyte setup is out, but it’s for the current HDOs, and has only been met with mediocre reviews. Plus Caddx just came out with the Vista, a DJI camera/air unfit that’s smaller and lighter than DJI’s original air unit. Just my two cents though, happy flying!