What vs How and when to work on each?

I’m PO with a Scrum team. We build and maintain a B2B service that is in a rather complex environment. We have always struggled with being able to estimate PBIs without delving into a lot of the How. When we focus on the What in PBR and leave the How for the sprint, the results are not always great. The team often wants to run planning PBIs a sprint prior to doing the actual work, which means everything takes a minimum of two sprints.  I feel the design is part of the development process and is therefore part of the PBI.