I also find processing 35mm in 3 gallon tanks a tricky business. I have no great problems with 120 or sheet film but with 35mm, there was a tendency to get surge marks. As I work as a printer, getting the processing right was always very important, so I decided early on to only process 35mm in hand tanks and the rest in deep tanks. Apart from my own film, I rarely get much film from clients to process these days, (thanks digital), but still prefer to do up to 10 at a time using two 5 reel tanks, one in each hand, and all the jugs of chemicals ready to use. I did look into different methods of agitation with the 3 gallon tanks, and in most cases the processing was fine, but as I could not be sure, stopped processing 35mm in them myself.

However, a good friend who worked for me for many years used to do a lot of the processing, and she always got excellent results using deep tanks and I think would have been using the recommended Ilford way. Agitiation would be lift and tilts though in one of four different directions each time so the exact pattern was not repeated each time. (Same way I still do 120 in deep tanks.) Doing it over 30 seconds does sound very slow though. I think another thing we used to do was interspace each spiral in the cage with an empty one below so the developer could flow about better between the rolls when agitated. (And the fix too of course as lack of agitation in the fix can cause problems too.) OK, this cut down the capacity of each process but we still could do 18 rolls at a time, same as the the amount of 120 I'm happy to do. Also, this was the maximum we could get in the film drying cabinet.

Not much help I'm afraid but it is one of those things that can give excellent results when cracked, though sadly if it's wrong, it is very distressing so I do understand your anguish. It's surprising sometimes when looking at classic photographers work such as Robert Frank and seeing that they've suffered the same fate too. Not much consolation but at least we are in good company!