Since I did that in Moon and Ultra Sun (shiny Rowlet on my first SR both times), I will try to soft-reset for shiny Grookey c:
Sorry to ruin your fun here, but apparently they locked this time the soft-resetting method to get a shiny Starter. This also goes for the legendaries. This means, if you want a shiny Starter you have to breed one. :/
Here are the news, a dataminer found out about it: https://nintendosoup.com/pokemon-sw...iny-locked-starter-hidden-abilities-revealed/
Don't understand the logic here why they locked it but then again I don't understand many decisions GF made...