Like if I shiny hunt for Rowlet then I get it after 200 eggs will the shiny rate reset or can I go on to find another shiny with a higher rate ? The reason for this question is because I'm breeding for HA shiny rowlet and if it isn't HA then maybe I'll be able to continue.

When you are hatching eggs, the shiny chance doesn’t increase. You will always have the same chance of hatching a shiny, there is no “chain”. You could find three shinies in a row, you could find one after hundreds of eggs. The chance will never increase. The only way to increase chances are shiny charm and masuda method

No, your odds do not reset. Thinking it in terms of "lower rate" or "higher rate" is a bit of flawed logic when shiny hunting by breeding. Every egg you hatch has the exact same odds of being shiny, regardless of whether it's your first egg, your 1000th, or if you've just hatched a shiny.