Sounds like a couple other people were hitting the wall I did, so I came back to report my experience...
I could get the tickets I wanted up until 12/31 but the moment I tried for 2024 Dineyland tickets they only had a few ticket options, but I wanted park hoppers with
Genie+.
So I took a chance, and bought the 2023 ones for 12/31 assuming I could at least put the money towards 2024 tickets if all else failed. Luckily the site let me go ahead and make reservations for my actual dates. In fact I dont see 12/31 anywhere on my tickets at all! In an abundance of caution I chatted with a rep online who checked my reservations also and said I was good to go. The system should not let you reserve a day without a ticket that works for it.
YMMV, but I think I'm set! I'll be mentally prepared for a snafu at the gate but 95% confident I just saved $198 and another 5% through my card.