User testing, is it really necessary to do for each application in same domain

Just wanna start saying that hope everyone is safe and healthy. I have thought about user testing done for apps sharing the same domain to start the design improvement process. For example I have had 4 clients last year all into retail banking domain with very similar features. I just wanted to hear your thoughts on conducting the user testing each time with the same purposes for each app (purpose is to detect usability problems). Considering the cost of testing (time and budget) I believe with the know-how in hand expert review is also sufficient in this case. I ended up with detecting the same problems I've detected through expert review and feel like I'm just spending unnecessary resources and extending the timeline of the project. What do you think on this matter? How would you communicate it with your client?