I would caution against bifold if your doors will be very wide. We just replaced a pair of bifold door with sliding after the befold doors fell loose from their hinges for the umpteenth time, and I got tired of paying for their restoration,and worried that someone might get hurt. I think you can "dress up" sliding doors so their appearance is in keeping with the Victorian theme you want to keep. Just remember that, in those days built-in closets were very rare. People used wardrobes or armoires to store the few clothes they had.

I would look at white plantation shutters for the windows and a Vertiglide on the slider since you use it every day. I would then either add a cornice over the Vertiglide or some drapery side panels. Now if you have young children I am going to change my Vertiglide recommendation to a vertical. I know no one like verticals but if you have sticky little hands, you want to be able to clean the blind easily! When they have grown to the point where you don't have to worry about that anymore, then you can move to a Vertiglide or something else. Here are some links to a blog you may want to look at for ideas. https://dressthatwindow.com/beauty-of-shutters/ https://dressthatwindow.com/i-hate-verticals-what-else-is-there-for-sliding-glass-doors/ https://dressthatwindow.com/drapery-side-panels-complete-the-look/ Not a perfect example but just trying to give you a sample of the combination of shutters, a Vertiglide and a cornice board. You can really dress up the windows on that back wall to look great! Good Luck!

We just went through very much the same design question. Back of our house has greatroom, kitchen, and morning room in a L. Greatroom is 20'4", leading out to deck/lake. Originally stained double doors and bank of 2-3 windows on each side. Looked very classy. We looked then at the Western corner slider because we love the seamless look (https://westernwindowsystems.com/multi-slide-door-systems/series-600-multi-slide-aluminum/). Turns out there's a lot of cost for the doors, and the engineering associated because of the removal of the corner. So we ended up doing 1 long slider and 1 shorter slider meeting at the corner. How this is related to your question is the long slider. We had options of doing 4x4' like you, but ended up doing 3x5'. Mainly because we wanted the doors to stack to one side, rather than open in the middle. 4 panel would be more room lost on the interior of the house. Originally we were going to do a pocket, but there wasn't enough room on the wall for 3x5' + pocket, so now we are doing one stationary panel but it will open to 10'. From a design perspective, I have to admit that the house looks different from the outside. I love the view out it gives, but it was a bit jarring with the craftsman/sized windows we had specified. Have you looked at a 3D model to see how the sliders flow with your architecture? Oh and be sure to check how much the bug screen is. The bug screen for a Western Windows 3x15x8 was almost $2,000! :/

'm not sure how to help you get the door out, but I would be concerned about the sagging frame. We had a similar issue - caused by a leak under the door, which seeped into the wall below. The door had to come out and the wall repaired. It was a pretty big job, but it would have been a lot worse if we hadn't fixed it when we did. And yes, we replaced the door. I hope this isn't your problem with the door frame.