I've had the almost identical problem, and have run the gamut of electrics over the years. I actually prefer a foil for speed and precision - and the Series 9 in my opinion is Braun's best - but I found the rotary Norelcos give me the least irritating shave regarding ingrown hairs. I would add that the most recent Norelcos with the "frosted" rings (S9000 Prestige, 7000 Series) seem to be the most effective at this, and it really does take about a month to adapt to a rotary. I also use scuttle, brush and lather, full up just like a DE shave. Just like a DE, equipment and technique matter for your individual shave, and if you've reached the pinnacle with the Series 9, shave on. I just found that a rotary reduced my rate of ingrown hairs with continued use, my theory being because of the way a rotary cuts - leaving the tip of the hair shaft rounded and "even" as opposed to the clean, sharp cut of a foil (or a DE blade, actually). And of course, YMMV.My neck hair grows S to N in the middle then S to NW and S to NE on the sides and all of it lays so flat to my skin that I can get an ingrown hair during the first couple weeks of letting a beard grow. An electric has become my only option for neck shaving and even that leaves me with a few small bumps. I've had a cheap Remington, 2 different brain series 3s, Braun series 7, Braun series 9, and a skull shaver pitbull gold. The Remington and the skull shaver are junk. The Braun series 7 and series 9 are fantastic but I'm not a fan of the pivot and the motors are almost too strong so they cause irritation. The Braun series 3 is my favorite. Just strong enough to get the job done but not overly rough on my skin.
My problem is the same I've had forever: nothing stops ingrown hairs on my neck. Even shaving just WTG has caused them while also giving crappy shaves (my hair is so dark and thick that I have a shadow even at BBS). I've been at traditional shaving for several years now and am familiar with my grain map and proper prep and beard reduction, I've tried way too many razors and blades, and even more soaps, creams, and aftershaves. I can grow a very nice full beard quickly and I work in a career path where a beard is acceptable, I had a long full beard between 2011-2016 and have had one off and on since then, however I hate the way a beard looks, feels, and maintenance involved. My wife likes BBS or full beard, nothing in between, causing me to chase BBS everyday or to not shave at all. I also shave my head (by choice) so DE shaving head + face to BBS everyday is 30-40 mins I don't have so an electric has been great and I really only pull out a blade for "date nights" but I still can't shake this neck irritation. I have a pic below that shows my neck growth pattern, just as reference, and fortunately my skin is calm as we speak because I haven't shaved my face/neck since Saturday night. This is only 2.5 days growth: View attachment 961933