The point of "DSi downgrade" is to be able of downgrading from system ver. 11.0.0-XX without a hardmod. @fikatr Have you tried to use your entrypoints adjusted to 10.7 (the NATIVE_FIRM version) instead of 11? And I dunno the process but AFAIK if you use super mistery or basehaxx you should perform some kind of process to adjust the RAM's contents. (I may be wrong since I didn't re-read the guide properly since last month).
Have you tried to do what the guide suggest? I literally just did this twice in a row on a N3DS and an O3DS. It clearly states and I'm paraphrasing here, "(PLai)sysUpdater WILL freeze at HAX Init when using secondary exploits such as SMHaxx, Bashaxx, etc. The fix this you have to load a HomeBrew (Any HomeBrew) and return to the HomeBrew menu without exiting. THEN run the sysupdater to downgrade".
EDIT: "If you are using supermysterychunkhax or smashbroshax as your entrypoint, you should open any homebrew application (such as menuhax_manager) then close it back to the Homebrew Launcher (which will fix memory contents). If you do not, svchax will not init properly."