"Flashbacky" transition in Hitfilm
A very simple one you can do is a transition that is dipping to white. If your editor doesn't offer this kind of transition you can do the following: add a still image which is all white, do a cross dissolve from scene A to the white still, then a cross dissolve from the white still to scene B. Ideally the first time you use it is when audience can infer clearly from context that scene B is a flashback, so that they recognize that this type of transition represents a flashback. Also you may add a subtle, short sound effect.
It does not look that fancy, but should be easy to do and many would argue anyway to keep your transitions clean and simple anyway.