Great rant , you care , that's even better !
Had reef tanks before you know about cycling that helps a lot !
I got you on the access .
I used Birch when I built RC airplanes in my younger days it cost $$$. What about plexiglass would be cheaper . Have any old tanks around cut the side out so to make use of the length more , use the eggcrate as a ramp all done. That would be clear to see better and no sealers needed to water proof. Just a few ideas for your next ATBA ?
Turtles climb very well , make it escape proof , long way to the floor.
I'm an old retired man and a pack rat myself being on a fixed income. I also keep strap everything. At times I think it's more of an adjustment for us than the turtle.
Since you are handy with DIY that will help save money. I use several price trackers , helps too on your wish list. Maybe one day down the road for a fixture ( to light up the tank) look into those new LED strip light's, are very narrow and light up well to offer a larger ATBA. But you still need a UVA and UVB over the basking dock or one that's does both like a Mega Ray.
A RES is hardy you don't need to rush for everything today. Watch sales carefully and use those price trackers. Extra planning helps so you don't buy inadequate things. Look into making your own filter , a bucket and pump with good media works very well for only a few dollars. Google for ideas. Most filters are sold for fish , a turtle will need 3-5x's the flow of fish . Also most come with media for fish , see media threads for better setup for turtles. Filter with a large media area will only help on the mess a turtle makes. I keep my filter with mostly bio media and a pad on top to keep it clean along with a cycled tank for Piggley , works for us. Stay away from filter's that only use pre made thing you always have to change. Buy bulk and make your own with media bags to fit a turtle needs is better.
It will come together and become very easy for the both of you. Study up and plan things out one at a time if you need to. Use that experience / creativity from your reef tanks. You are already handy on the DIY side . You will be a great parent for her and enjoy many years together. Turtles are Great pets/friends/kids ! Don't go insane

observing and patience are a big part in keeping turtles. She's already in a better place with you.
Sorry I just saw your other post you attached to anothers thread , see reply.