Wow, that's a lot! Let's go over this in order:
I have no idea the Prime Day deal will be on the Z Fold 3. Read the trade-in terms and conditions VERY carefully! If you send in a device that Samsung determines is in a condition that does not meet the terms, you get charged full price and they will not return your S10+! If your S10+ has any screen cracks at all, Samsung will reject it. If it still works, you're probably better off selling it to someone for cash.
Keep in mind that we are roughly two months away from the Z Fold 4 - while it's said to be an incremental upgrade, it will definitely have features that will blow away the Z Fold 3. My son just bought the Z Fold 3, but only because his broken, cracked, barely functional Note 9 was probably not going to make it another two months.
As far as the Fold Edition S Pen goes, it depends on what you would do with the S Pen and how often you would use it. I got the Fold Edition rather than the huge premium S Pen for portability; and I really wouldn't use the extra Bluetooth features very often. I rely on the S Pen almost all day, every day: so it was important for me to get one.
HOWEVER: do NOT get the Samsung Flip Case! It is a nightmare of a case that will likely aggravate you to no end. I bought the Spigen Thin Fit P case... and while some people don't like the idea of using adhesive strips holding it in place, I love this case and have no intention of ever removing it. The S Pen is right where I can get to it; and the front display is accessible.
I hope this helps!