Screen porches are often used. If you have glass, consider converting it to screens. The simplest thing to do would probably to to put deck over you planter boxes. It would extend your deck. Then, you could put wide stairs from the new deck to the yard. You'd improve your flow, have a bigger deck and it would probably cost considerably less than removing the planter boxes. You might keep them in front of the sun room, or not as you choose.

I would remove the planter.

Save the pavers for a new patio on the ground, or sell them and use the $ to finish the clean up.

Honestly, your hardscape design looks well done to me. Your house is higher than the grass so the planter wall makes a nice transition so that you don't have an unmanageable slope. Just landscape the planter area with low-maintenance plants! It could be really beautiful with a mixture of low shrubs and colorful perennial flowers. The key is planning for low maintenance.

The best way to make your screened in porch / sunroom functional is to choose comfortable furniture that fits the space. Even if you had all deck instead of the sunroom, you would still probably want to seating areas. This just means you have a nice seating area to use when the weather is cooler or during precipitation or when the bugs are out. Embrace it!

Hire a landscape architect/install firm to envision and design the space for and with you. It’s well worth the money if you can swing it. I agree that the deck needs a redo, but I’d probably try to work with the planter. Regardless, a designer can provide you with a beautiful plan and you can go from there.

I think you already have some great elements, it's just a matter of making them work for you. Since the sunroom is already on decking and you find it unusable as is, remove the sliders and windows and open it up so you have a long section of deck which is partially covered, partially open. It looks like you have a storm door which will still keep the house secure. Add a furniture grouping under the covered portion and maybe a weatherproof dining table or lounge chairs in the open section.

Adding low plants in the planters will soften the hard surfaces and add color and texture. You can consider a rock garden with low maintenance plants if gardening isn't your thing.

Your rock wall is beautiful and will make a great back drop as you sit on your deck and look out. The beds are oddly shaped and the grass needs serious attention but there are lots of possibilities to have larger beds and/or another seating area or fire pit, fountains, etc.

Different elevations (within reason) add a lot of charm and interest to garden areas, I think the deck and planters can be used to your advantage.

Also, your yard is sloped - if you remove the sunroom windows and add flagstone or stepping stones in the planter area, then more steps to the grass, you'll gain another path to the yard and open it all up.

Just an idea...Here's a visual..... Deck over the entire flower bed and add a railing with attched bench seating. Do away with the doors and screens, you'll be able to place your grill underneath the patio cover so you can grill when it rains. I would also do away with the stone surrounding the trees in the yard. Let the the grass grow around the trees for a cleaner look and not have to deal with weeds and mulch.

I love deck so IMO just get a multi level deck designed to serveyour purpos for outdoor living .BTW I hate bench seating all built in