How should an experienced hire position prior industry expertise without getting pigeonholed?

Well, in a way you've sort of answered this question yourself.

It's clear you're thinking about how to walk the line here - and that's exactly what you need to do!

Now, in terms of writing and answer to you, it's really really hard to articulate how to walk this line. But, in essence, you want to:

Highlight and leverage your past experience, but focusing more on the skills built (i.e. leadership, problem-solving, etc.) than on the financial expertise acquired

I did this for my IT background (i.e. pivoted most questions to how I had to solve client problems when building the system, I had to work across multiple teams, I had to stick to timelines and manage people, etc. etc.)

In terms of expressing your future interest, it's best to both emphasize that you like a wide range of things and are looking to explore options but are also particularly drawn to x (whatever x is is perfectly acceptable). This shows that you're both a "team player" and won't complain about projects, but also that you have drive/ambition and will eventually align with a practice area

Hope this makes sense! Honestly, the best way to prepare for this is to simulate this (with friends, colleagues, or a coach) in a mock fit interview. As you practice your answers, you'll naturally be able to identify what is a good answer versus a bad!