"My brokerage told me I didn't need to separate them out like that."
While there is no benefit to doing the conversions to separate accounts, you still must track the amount of each conversion separately.
"Now they tell me that when I go to take a distribution from *any* Roth (even the one I've had since 1999) I will have to fill out form 8606 to see how much is taxable."
For distributions made prior to reaching age 59½, that's correct. This has always been the case whether or not you have multiple Roth IRA accounts. However, since you are over age 59½ and your first Roth IRA contribution was for some year prior to 2021, distributions made from any of your Roth IRAs in 2025 and beyond are qualified Roth IRA distributions which are not reportable on Form 8606 or Form 5329.
5-year conversion clocks no longer apply after age 59½. The 5-year conversion clocks are only used to determine if a Roth IRA distribution is subject to any early-distribution penalty, and you are not subject to an early-distribution penalty on any Roth IRA distribution made after age 59½.
"we're looking at possibly drawing on my oldest Roth in 2029"
For tax purposes, it makes no difference which Roth IRA a ordinary Roth IRA distribution comes from. You've already satisfied the 5-year qualification and the age 59½ requirements for your own Roth IRAs.
