Highland brings up some great points that I agree with. I definitely am learning as well that for some clients that are larger and has an in-house team that you support as an SEO, they might just want you to focus on SEO. But if it's a smaller company you're working with that maybe doesn't have their own Director of Marketing, then you kind of want to take on that role and provide a variety of services. It's kind of a business-dependent approach, meaning every company has different needs - no two are the same. Although productizing the services for clarity, ease of pitching is a good idea too.
In response to your question, I would agree with you that it is a difficult situation - writing content for a law firm that you don't have the technical chops for. Assuming that you're dealing with lawyers - I would imagine they are super busy and their billable hourly rate is so high that they're incentivized to work on their own client work rather than help you write content.
Perhaps rather than a blog, you can organize and repackage lawyer-approved information and helpful guides to post on their site. Think about who their clients are and what the clients need help with and serve them the best info possible on the site. Showcase the trust and authority of the firm with evergreen content.
If you do decide to continue blogging, perhaps take the Movado approach - where in their real estate niche instead of talking about real estate jargon and industry issues, they broaden out to talk about cities, neighborhoods, and local topics in cities across the world.