...methods or processes to better manage these clients' priorities...
There's no silver bullet and part of that is the ephemeral nature of priorities. They change over time and what's relevant now may not be as important in 9 months or a year, or whatever the time scale is for your clients.
That said, it's good to at least document things as snapshots of unfinished "wish list" items. Essentially, a backlog of all the things:
Indeed, it contains all things. If an item is in there, it might get done. If it isn’t, there is no chance that it will get done.
It’s a list of ‘want to do’ items, not a commitment. Items can be estimated (preferable) or not, but neither case implies a specific time commitment for completion.
Just take everything you or anyone else wants to do and stick it in a backlog.
Any scope of specific considerations can be managed as a backlog, with someone acting as it's product owner. I have trouble thinking of examples it doesn't apply to.
Let's use "e-commerce UX" as an example. An e-commerce UX PO might be on the hook for delivery of value to the business units via all things impacted by e-commerce user experience, and they'd have a heap of these "wish list" items described as coherent user stories in their backlog ordered by some informed priority. This PO is responsible for managing that backlog of effort and rationalizing each item in terms of its potential to impact e-commerce UX.
The PO is responsible for balancing what everyone wants done next against what everyone has also agreed to as being important right now.
A framework like this might be an appropriate solution for you, since efforts and priorities are constantly confronted with fair questions about delivery of value all along the way.