This is also similar to what happens in waterfall between releases when the product manager is trying to launch while also pumping out the next MRD. The problem is just too much stuff to do at a point in time than can be supported.
Here are 4 generic options:
1. Reduce the amount of work - this is likely not so easy to do, or not desirable.
2. Phase the tasks - while you can't really change the timing of the tasks, maybe you can change the structure of the team such that some are focused on the current sprint while others are doing the planning for the next sprint and then swap as the sprints progress.
3. Specialize within the team - another is to develop specialization within the product management team such that all are not product owners, and some are pure product managers/product marketers doing more of the research and outbound tasks and you have a matrixed organization.
4. Add dedicated resources - Lastly, consider a hierarchical product owner team, and add business analysts or more technical-focused product owners having day-to-day responsibility working with Dev, and Lead Product Owners who are overseeing, but only needing to spend 30-50% of their time with the team and the rest of the time doing planning and are externally focused.
The solution is not obvious and there are multiple alternatives, but all have trade-offs, however if the current situation doesn't work so well.....