2010 Tundra 5.7 Grade 2wdr

Hey All,
I have a 2010 tundra crewmax 5.7 2 wheel drive with the 6 speed auto and 248500 miles , its the Ab60E transmission. I am going to trade this in soon or sell it and need to resolve this 1st. A Little History - My issue started with the truck sometimes taking about 30 sec. or so once shifted into reverse for the reverse to engage when I first started her up in the morning. You can feel it shift into gear not like its slowly slipping into gear. Drive shifts fine through all other gears. Then a month or two after it started throwing a code for transmission filter low transmission fluid. I leveled the truck off my bed top and a framing level. I drained out the pan, measured the amount. I put a pin in the oil cooler valve to hold it open, Dropped the pan and replaced the filter and pan seal and refilled with new toyota fluid. I used the trick with the paperclip where the truck tells you the correct temp to pull the plug and I waited for a super fine drain stream out of the tranny like everything I had read. that was 3 months ago. Since then it has popped a P0894 and a C1201 code a couple times and i pulled the ground cable off to reset the codes so it doesnt go into limp mode. I havent needed to drive the truck very much the last couple months so it wasnt a big deal but now im starting to get it ready to sell and need to get it fixed.
So as of now the reverse is intermittent. Sometimes wont go into reverse until it I let it run for about 10 min. I thought it was just some mornings and after setting all night but I pulled into a parking space and got stuck for about 10 to 15 min after I came out of home depot the other day. So now i just make sure to back into a spot.
I was going to try and add another quart maybe of fluid but it makes me nervous of overfilling. Or replace the S1 solenoid but at a couple hundred a pop and the labor involved i was looking for some insight before i spent the time. I will probably end up taking it to a professional but its turned into a challenging puzzle I need to solve. LoL Any help greatly appreciated.