Joffo mate I think you need to look at your settings. That sounder and transducer combo is more than enough to be reading good bottom at 120 to 250m. The fact that you can hold bottom at speed up to 100m tells me he tranny is probably set up properly. There are some sounder gurus on here that may be able to give some technical help but.
Next time you’re out that wide, try putting the range and gain to auto and the zoom to off. Then have a play with the settings from there. Once you have bottom, switch between 50hz and 200hz once you have some structure to highlight the differences and working on your zoom/bottom lock and gain settings from there.