Adding an update. I don't think I have this sorted. After a few more weeks in 15 psi, I did some testing this weekend, and with 5ft lines I got a lot of foam and beers that are flat. Thought maybe a bit over carbed, so added a longer 9ft line of 4mm Eva barrier to one of the taps and this reduces the foam to about 1-2 fingers, but the beer is still no where near beer carbed to the same level in bottles according to the tables and it's pretty flat within about 5 minutes in the glass.
Serving at 10 or 12 psi is about the same outcome. It's not that different to where I started from but somewhat better.
I've done a few pours from a couple kegs, giving the taps and glass enough contact time to chill down (plus get half drunk tasting it all).
According to the carbonation charts the beer should be at the higher end of the usual range for lagers, nothing crazy and I didn't force carb. The line length calculator I used gave me 5ft as the ideal line length for 12 psi serving pressure. So am a bit lost at what to try next.
I didn't get much head on the beers previously which was about the only difference.
Try sobering up, cutting 30cm of the line at a time and trying again? Bleeding the kegs a bit to reduce carbonation? Will it sort itself out once the kegs empty a bit, have seen that mentioned in some discussions of similar issues?
ETA: did as bit more reading and might be that I'm reducing the pressure to serve, causing bubbles in the tap lines because there not enough pressure in them and crashing c02 out of the beer, so maybe don't drop pressure and use longer lines. Will test that next.