This is certainly possible.
I have done it A-spec (I, like you, aimed for 100% completion in A-spec, and I did it!) and it wasn't terribly hard, although, obviously much harder than, say, the Sunday Cup!
I was able to win all races in Formula GT Series using FGT car with minimum downforce settings, in order to achieve 127 A-spec points per race.
I cannot remember my exact settings for Nurburgring, since my FGT was then setup for the races after that.
I do recall the following though.
Downforce set to minimum. - This will certainly
not help with your tyre wear dilemma though. Would recommend full downforce settings.
Suspension was softened to suit the bumpy surface, and ride height was increased for this reason too.
Toe angle at the rear was decreased to -1.
Camber angles were increased slightly.
Brake balance set to 6 Front, 5 Rear to protect rears from locking up & wearing.
ASM settings 0
TCS set to 1 or 2 to prevent too much wheelspin.
LSD settings I can't recall.
Transmission set such that you don't quite hit maximum revs in top gear on the long back stretch towards Antoniusbrucke.
I pitted at the end of every lap, didn't win by miles & miles, but I did win, and pretty much cleanly. You need to make near perfect laps, leave the track a lot and your chances of winning diminish.
The fact that the AI stay out 2 laps hurts them more than you, they get into the same situation that you do after 3/4 of their 2nd lap, tyres are shredded and they spend the rest of the lap pinging off each other, sandtraps, walls etc, which allows you to make up the time you lose in the pits. Eventually the time your gaining will exceed the time they lose in their 2nd laps and you'll pass them, coming home to win.
Try to figure out exactly how much fuel you need per lap and carry only a couple more units than that (for safeties sake!) after your tank initially drains. Lower fuel level = better tyre wear.
At the moment I have the PS2 paused about 1 hour into a 4hr Enduro, so I can't retrieve any of my exact settings or fast lap times.
I'll come back to this later when I'm done with that race.