The long simmer is new this year, we have a large electric roaster. Previously we had done smaller batches in a pot on stove.

Funny you mentioned salsa, we do that as well. Never bitter or tart there. It also cans well but you'll lose some texture. I add extra cilantro and lime juice when canning as those flavors tend to disappear during processing.

We used to blanch and peel the tomatoes but used to leave the seeds and then strain things after cooking. This year we milled because we thought maybe we were getting the off flavors from the seeds. No difference noticed with either technique.

Oddly enough, we never have this problem if we used store bought canned tomato products, puree , paste, juice etc. So something we're doing wrong.