Try not using your scuttle. Go with a face lathering to try and dial in the soaps best qualities. Too often when I started I was too focused on getting a bowl lather. After all I wanted to use every weapon at my disposal trying to get the best shave possible, and thought using my bowl had to be part of my arsenal at all costs. What it cost me was disillusionment, I wasn't getting the best my artisan soaps could provide. Once I got the face lathering down, hitting the mark for a given soap and that buttery glide, I got great shaves every time. Then once I knew about how thick or thin to lay it on I was better armed to create a great lather that worked best for each individual soap. They don't all work the same, even if your bowl lather looks the same, some work better thick and foamy whereas some work better thinner and more buttery. Give that a try, it's the only variable I can think of that you've not mentioned. You should be getting outstanding shaves using Synergy especially, or any good artisan soap for that matter. Menthol definitely has nothing to do with quality of shave, even on a subconscious level it wouldn't matter, even if you just miss the smell. Wet your face, load only your brush tips not trying to load the whole brush, begin painting your face until a nice creamy layer develops, redip just the brush tips once coverage is achieved and use the brush tips to work up a face lather that's going against direction of hair growth lifting hairs and looks almost like you've painted your face with buttercream frosting just a bit airier. If you start seeing bubbles you've likely used too much water, add a touch more product to brush tips and again just brush it on. Too little water, too thin a layer, that will be a dry shave and semi-clog a DE razor. Should be a happy medium, between lather and butter. Once found you'll have a much better idea what consistency to look for when lathering with your scuttle. You don't need peaks in a lather to get a good shave, some soaps perform better buttery.
Seems the only ingredient that's missing from your formula is finding out how this soap fifers differs from what you've been used to using. Works for me, each time I try a new soap I expect some differences in how to balance them and get the most from them.
I hope this helps you too.