I'm a lifelong Technician, but I know situations and problems differ a lot sometimes. I've had this very issue. It was my transmission toasted that caused this issue, I bought a little time by adding a container full of brake fluid which causes the internal seals to expand and make things work better for a little while. But it's no fix, maybe yours isn't shot, but I'd try the brake fluid as it can't hurt. Mine was the same way, like it had a 4000-5000 RPM Stall Converter and didn't engage 1st Gear or move at all u til 4K-5K was reached. A week later I lost all forward gears and then had to drive home in reverse which thankfully wasn't far but far enough! It sounds like you've changed fluid since you've changed the solenoids in the pan, if you didn't change both the fluid/filter you could try that but I'm sure you did, be sure to drain and change the fluid in the torque converter too.
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Did it have the good old "Burnt" smell when you drained ir have smelled the transmission fluid. Add some brake fluid or Lucas Transmission Treatment to see what you're working with. After I did so, it dropped right into gear at a normal idle speed like everything was perfect, I was changing It about a week or so later. Maybe you'll be luckier than I was- but I appreciate honesty and thought you would too. I'd start looking for a good used transmission or someone to build one. There's an add on the Greensboro, NC craigslist for a guy that builds bullet proof/upgraded transmissions guaranteed not to fail for about $849. Even if that's not close by, it may be worth looking into or finding a similar situation closer. Shipping is pretty cheap these days, it may be a great solution.
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Thanks again and have a great day!