Welcome HappyMrsCope! I second a lot of what mnmom23 said. I will add my 2 cents based on my experience, with the caveat that what worked for my Jan. bday kiddo may or may not work for any other kiddo.
Whenever I hear "GT program doesn't start until 3rd grade" I have to comment.
Our school district had the same policy when our DS started K, but we were able to get some acceleration, differentiation, and compacting for our son in kindergarten by working with the GT coordinator, the teacher, and the flexible principal. For us, we wanted our son to go to kindergarten with agemates because he refused to learn to write, even though academically he was years advanced. Because the school was willing to differentiate and work with our kiddo, kindergarten was OK. It was not without downsides, though, as they wanted to "get to know" our kiddo for themselves, which took a really long time, since DS is really quiet and not showy. Finally they saw how far advanced he was when they tested him on the MAP tests, which they normally gave starting in 2nd. Because 1st grade is a "learn-to-read and do very basic math" year in our district, we successfully argued for a grade-skip even though DS's writing skills were not all that great.
As for being the youngest in the class, DS with his grade skip was the youngest in his class, sometimes even by 2 years because of the red shirted kids, and among the smallest, but he did not mind. I would agree that if sports are important in your family you may want to reconsider having your child be the youngest. That is not an issue with our kiddo, who is not athletic.
Do you happen to have any GT schools in your area? I'm guessing based on your questions that you have at least one qualifying score for DYS, which makes regular school placement trickier. It can work, but you will likely spend a lot of time advocating, and you will likely have to look into grade and/or subject acceleration. We have found a good fit at a school for HG kids, an accelerated program. DS has a classroom full of intellectual peers, and while there is still a range of abilities in the classroom, it is a GT range, and it is nowhere near the giant range of abilities you would find in a regular classroom (which makes it much harder for the teacher to work much with the kids doing well).
From your "cheers" and "mum" comments, just want to verify that you are in the US. Unfortunately, DYS is only a US organization at this point. But this board is always here, and hopefully we can provide some useful info. Please feel free to ask away!
