The biggest problem I see, is the cost. With enough money, you can do anything. What I gather is this is a single speed steel framed flat land city bike that is being retooled to jump on the latest trend (gravel) and surprise, surprise it is having difficulty.

Anyone who could do what you want is going to charge you so much more than a new frame costs it just wouldn't be worth it, either for them to do it, or for you to have it done. It's not a simple job by any means. To be done properly, we're talking a whole new rear triangle. That costs real money, more so since the builder is starting with a work already in process and has to eat the liability of others work & complications of their quality control decisions...and this is coming from a guy who has gleefully thrown good money after bad for some pretty outrageous projects.

Simply put. Tell your significant other that it costs more to modify this bike than it does to obtain a new frame.

Below is an old 1997 Trek mountain bike gravel bike conversion. The conversion cost for new bars, shifters, cables, housing, bartape was under $250...shifters being $175 of it. It currently has 559x47c mountain bike tires on it (pictured). But, Rene Hearse Naches Pass branded GravelKing tires cost the same $71 dollars as heavy wooden Schwalbe Marathon Supreme and ride with every bit of the glorious smoothness you would expect.

My point is, you can accomplish your goal, have a lot of freedom, and a much more capable machine at less than half the cost of your current frame alone if you are even the slightest bit resourceful.
All-terrain warbike by Richard Mozzarella, on Flickr

You may even simply re-tire your old mountain bike with https://www.renehersecycles.com/product-category/components/tires/26-inch/ and be money/capability/speed ahead of where you are now.
(Rene Herse tires are made by Panaracer & since you already have Gravelkink SK's you know how good they roll. Moreorless they are made on the same production line and essentially with very little difference, each brand gets every other size, unless you choose the "extralight" which is Rene Herse exclusively.)