You've got several issues with the plum tree that I can think of. I am not certain of the exact PH etc for the soil, but you can easily find that information.
Hubby says that it is better to plant the trees while they are young, and that transplanting them is worse the older they are.
Your main problem is the bad soil. If you do what most do, and simply dig a hole slightly bigger than the root ball of improved soil, and transplant it, down the road the tree may have problems. Roots will be gathering nutrients further and further away from the tree as it grows.
You will need to have a sizable plot of improved soil in order for it to succeed long term. Maybe some of the others will have some additional hints. Hope this helps.
The lettuce is called Great Lakes #118 and is my first attempt so I guess I’ll just keep watching to see how they turn out. If it’s a complete failure I’ll reuse the pots for some other veggies or herbs.
And thanks for the advice on the plum tree. I’ll skip on using a pot and go directly into the ground. Had a plum tree already established in the backyard years ago when I moved in (until termites killed it) so maybe I’ll look at planting the new tree where the other one was and dig a big hole to add lots of rich soil so the roots will have a good chance at thriving.