I see Jeanne's point - For Heifer #3 if you are committed to finishing her out, might want to check in to see if Castle Meats in Ione has a slot available for Nov/Dec. They are usually booked out several months but have had a number of cancellations this year. Jethro at Castle does a fantastic job cutting for us, we use Tom and Thomas at The Meat Shop in Martell as well as Jethro at Castle Meats as part of our freezer beef program.
I don't know much about Dexter influenced genetics, but from what little I've observed they will finish small regardless of the amount of feed you put into them. I'd think you'd still want to get her to 1100 lbs live weight or better though, and that will take some time. For your others based on what I'm seeing in the pictures, I would be waiting until next year. I'd make sure they have been wormed and see how they respond to a transition into a finishing diet. If they don't start converting, you might want to consider cutting your losses on them and trade out for some that will finish either more quickly or put you in a better position to hit the January harvest. Feed prices are so ridiculous in California the grain finished cattle need to produce serious pounds for their inputs or your costs will quickly outrun what they can bring as freezer beef.
FWIW If you have some buyers locked in that you need to fill orders for, I do have a couple good BWF steers that I expect are high 9 weights running on irrigated pasture that are ready to be pulled off and start finishing, they would hit your mid-late Oct goal based on our past feeding regimen if started right away. I just need to check with a friend who has first option on them whether he's planning to take them.
Another option - I watched some steers go through CLM this week at 10-11 weights that went very cheap in my book - They only brought $100-200 over the 6 weight steers I pulled straight off the cow and hauled in, and were nice straight black angus steers; someone just made the mistake of holding on to them too long in our goofy California market. Those would have been ready in 90 days no problem if pushed. I culled one 1200 lb 19 month old heifer this week, she was my only one that preg checked open. She brought only $100 more than those 6 weight steers I sold, but I didn't have a butcher slot for her on the books. I'd think if you were willing to do some trading, you could fit whatever works best for you and your customers.