sorry I'm not back from the dead yet but when I am we'll do max raids online miss you all
When a Pokemon uses Transform or has the ability Impostor, it will turn into the Max Raid Boss. It will not immediately Dynamax, however.
You cannot use Transform on a Max Raid Battle Boss which has it's shield up.
If you're facing a Gigantamax Pokemon and Transform into it, and then Dynamax, it will be the Pokemon's regular Dynamax form.
Source (Poketips Video)