I can confirm a lot of what has already been said. I've used spacemacs since 2017, have contributed to it, and have written several private layers for it, such as a custom org layer.
But I switch to Doom emacs literally two weeks ago and I'm never going back. I find doom to be much faster not just startup time but in general responsiveness (one major exception: hitting SPC and waiting for key binding tips to show seems to be much slower in doom than spacemacs). Other than that I found doom to be much easier to configure and much more configurable.
The major selling point for me was that in doom emacs there is much much less ceremony in wrapping a new package in a module, and more importantly it is _easier_ to reuse documentation for those packages because most packages provide configuration examples using `use-package` anyway. My only hangup was less documentation around writing custom modules which took me about a day to figure out (I would advise to just read some of the source code or mimic a module that doom ships with).
YMMV, but if you feel like you've attained a certain level of emacs maturity, I would 100% recommend trying it out. Be prepared that some modules, like the haskell module, don't ship with a lot of keybindings so you'll have to set those up yourself.