I don't think there's any real bad long term effect from repeating kindergarten - IF your child can get past the stigma of flunking a year of school. For some, that propels them into a self-fulfilling prophecy of believing they are bad at school. If you go with putting your child into kindergarten now, I suggest now, before they start, have a conversation with them about it, that they are entering it a year ahead of many children and that you expect them to repeat it once when they are the same age as their classmates.
I don't want to call it brainwashing ... but that's what it is. Brainwashing them into understanding that what they are doing is advanced, and keep that messaging up throughout the year. If they are struggling with a concept, you just keep repeating "Wow, I'm so impressed that you are trying this, when it's aimed at people a year older than you. That's like your classmates doing work from the first grade room!"
At the end of the year, either they move ahead and all is good, or they repeat the year, but they are prepped to spend the rest of their school career explaining (to themselves or friends) that they repeated kindergarten because they did the first year of it at a super young age - not that they were "too stupid" for kindergarten or developmentally slow in some way.
Another thing to consider is whether - once your child is in kindergarten - there is outside pressure on their teacher to inflate their grades or promote them to the next grade whether or not they are ready. There can be a lot of pressure on teachers to make it look like all their kids are succeeding. If children in the classroom aren't ready for an activity and can't master it yet, the administrators may evaluate the teacher as "ineffective" - even though it's more a statement about the maturity of the kids than the teacher. And if a school has to hold a child back a grade, that might be a thing they have to report to the state, which can effect their school rating, their funding, all sorts of things.
It may actually be easier to put them in another local school kindergarten if you have schools of choice where you live, and then transfer them to their final local school district, so the receiving school doesn't have motivation to push them through to first grade if they aren't ready for it. You'd have to do more research on the feasibility of that. If it's doable, it might also help remove stigma for your child to restart kindergarten in a new school, so they aren't faced with seeing all their friends moved to first grade, and all their friends aren't knowing them as "that kid who flunked."