SOLID, is just a guide line, isn’t a rule. Sure if you follow the SOLID you have cleaner code (most of the time ) but you can have more complexity code. Most of companies/people don’t follow SOLID, they use the S and the D.

When you are mapping depending on your archi, you don’t map DTO to persistence model. You usually map DTO to domain to persistence model. (Because domain and persistence model are not the same). Using extensions methods can work better in some problems, I have been using both approaches. Thanks!