Last year was possibly my worst for tomatoes in 20 years, 2 months of rain made the season even shorter than normal. My best producers were Parks Whopper, Big Beef, and Early Girl, all hybrid indeterminates of course. I did get a fair harvest and a good bit of canning done.
For heirlooms/OP, have you tried many determinate types? Legend and Bush Beefsteak are on my list to try. In my slightly limited experience at growing some of the bigger heirlooms, it may also take a bigger season, nice big green fruit in Sept. makes ya want to cry.
I have mulched and not mulched, in my garden mulching with newspaper and leaves/grass clippings boosted size and yield.