I don't think I've ever done a $70mil team before, but I'm doing one now for the WIS championship.

I'm playing in Oakland Coliseum (pitchers park), but my hitters are getting absolutely slammed by fatigue.  I took some guys with low ABs and pop (like '06 Manny and '90 Clark), so I expected to have to sub for them a fair bit, but even my leadoff hitter ('98 Rickey, 670 RL PA) is at 96% after 17 games.  Again, I know I took short ABs, but I feel I should have enough to spread around in a $70mil league, especially in a pitcher's park.

Is this just an early season thing or is 5,100 PA not enough to get me through a low cap like this?

not nearly enough to stay at 100% though you should be in the low-to-mid 90s which is playable. you need a few more PA at lower caps because you're facing weaker pitching. Ironically you can get away with fewer IP. WISC is notorious for fatigue issues.