Home Treasures would be a great option for you! They are all made to order and do six corner fitted sheets all the time. I think they even have a couple flannel options. More on reddit.com

There's a product called the Better Bedder, it's basically a giant elastic band that goes along the perimeter of your mattress and holds your sheet in place. Maybe something like that? It's not like the individual bands with the clasps that you put on the corners of your sheets which, in my opinion, work just fine. I've never had one come undone or hurt me. More on reddit.com

A 30” wide mattress is a jail cot size. You can get a mattress special ordered in the narrow size, my family’s mattress factory is making one for an old customer right now, but I would think twice about your size choice. A typical adult will be more comfortable on a 38” split King. If you are very tight on available width you can get a split CalKing with 36” wide individual mattresses. The other big advantage of the King size alternatives is that you can buy fitted sheets for them. 30”x80” is not a standard size. More on reddit.com

All of your reasons to have one are spot on. The issue with a split queen is that you and your wife will only have about 29" +/- 1" of actual width while in your respective mattresses. For a visual crib mattresses are also 29" wide. If you truly go with a split queen adjustable set it's likely you will pay at least the same amount of money as a split king adjustable set but you will have comparatively 8" less sleeping area per side in a queen vs. king set up. Plus getting sheets and protectors will always be an issue and more pricey. The difference over all in width of a queen to a (eastern, traditional) king is 16" the length is the same for both at 80". Put a sheet of copy paper (8.5" x 11") beside your bed or nightstand (with 8.5" side parallel to the side of your nightstand or mattress) on the floor on each side of the bed. This now represents a king size bed set. If you have a queen now and if you can comfortably maneuver in the room (doors, closets, windows) around that new width you can fit a king size. Split king versus queen you will be far happier with the extra space and the investment will be equal to, possibly less, than a split queen set up as some companies treat this as a special order or offer at the same pricing as a twin extra long mattress (1/2 of a king). Tempurpedic offers split queen bases. A simple search on Amazon will show you if others offer them. We actually do our best to talk customers out of a split queen for these reasons we share. They are a return/exchange waiting to happen. Me personally I am 32" wide from outer point of shoulder to shoulder so as a back sleeper I am exceeding the width of a queen. Of course I'm not you but if you are average to slightly above average it's likely you'll quickly use the space up. One last other thing to know if you do go split queen is to a avoid a foam encased mattress of any kind. Not that this feature is a bad one but in a split queen and even in a king application the denser/firmer foam rail around the perimeter also now resides in the middle/split of your bed. This means that in the middle you will have a far firmer area measuring 6-8" in width. This area will not be as comfortable as the rest of your bed. And lastly we always recommend in split set ups to avoid quilted top mattresses. They magnify the split and create a valley between you and your wife. Stick with non-quilted flat top mattresses. More on reddit.com