PCB photoresist method etching incomplete - only etched around borders of traces
I'm suspicious of your etching solution, but I don't think that's the problem in this case.
This sounds like you weren't agitating your solution enough, and the solution over the large copper areas got saturated and couldn't accept more copper.  If I'm right, then the reason you got a deeper etch around the traces is because where there was resist, there was a reservoir of fresh solution.
You need to agitate your solution continuously.  Like, if the motion stops, you're not doing it right.  The technique that I've always used for hand-etching is to rock the etchant tray back and forth, so the etchant gently swishes across the board.  You want the etchant to be moving with respect to the board, and you want it to mix, at least gently.
I'm also concerned at your reported etching time.  Did you control for temperature?  Maybe it's correct for that etchant, but with the ferric chloride that I've used, if you etch at too low a temperature it takes longer (if I remember correctly 30 minutes is way too long) and the etchant will undercut the resist, making the traces rough around the edges and sometimes cut all the way across.
Get an old ARRL Handbook (I know the process is in the 1976 book) and look in the construction section.  You'll see a motorized etchant tray under a heat lamp.  Take that as a clue.