So, regular moves turning into Legendary signature moves upon form change bothers me a bit. Iron Head turning into Behemoth Blade/Bash during combat if the Galar dogs are holding the items that change their form in combat is quite counterintuitive, and IMO it would have worked better if they had gotten an specific Steel move for their non-crowned formes that outright stated it would change, rather than the standard physical Steel attack.
However, that's not the worst offender. Kyurem, my dude, it has been almost 12 years. Scary Face turning into Fusion Flare/Bolt makes no sense. We no longer have the excuse of PP ups, seeing those will be reset upon transfering into new games. Just... go the way the other fusions work, and rework the mess.
Glaciate was supposed to be a failure at pulling a Fusion move; have that one turn into the Fusion moves. Keep Freeze Shock and Ice Burn as attacks that are attempted to be learned upon form change, and forgotten once returning to normal. Rotom was able to do that the gen before Kyurem was introduced. Necrozma can do that with ease. Calyrex has the entire movepool of its fusees at its disposal ONLY when fused.