Not sure if this constitutes “success”, but I’ve seen large improvements in the pool and open water since the beginning of this year. To spare you the details, I’ve averaged between 15 and 20k a week (4-5 sessions at 3.5-4k). I’m pretty young (25), but I don’t have a swimming background or really even an endurance background (wrestling/weight lifting in high school). My initial pace was probably around what you’re swimming now and I’ve improved to 1:30s rather comfortably. It took a lot of work and a lot of “oh my god I may puke” moments, but it’s definitely possible. Oh, and a lot of youtubing.
As far as technique vs volume breakthrough - my n = 1 is both. Initially, my method of swimming faster was just flailing faster - which was exhausting and ineffective. The only way to spend more time improving form is spending more time in the pool - which inevitably leads to more volume.
Brandon