I like your list and don't have much to add.
If you have a newsletter, in addition to your website, you can mention them there. I know you already send thank you letters, but just had to add that. If it's "pretty" they can hang it up at their business if they want to, I addition to their plaque.
Another thing that I believe goes far is for you, your employees and volunteers to use the businesses that help, when everyone can. And let them know that the reason you are shopping/supporting their business.
For example I once volunteered for an organization and for their biggest fundraiser of the year Duncan Donuts brought free donuts, bagels and coffee for everyone participating in the event.
When I was in the neighborhood of that particular Duncan Donuts I would deliberately stop for coffee or a donut, ask for the manager, and let them know that I stopped at their shop because they supported our cause.
#FundraisingandDevelopment