I think some people used to replace the springs with heavier ones off of a truck. I'd check somewhere like Rock Auto to see what they have. Probably cheaper than genuine Ford, but still good quality available. I think the F250 front springs were rated at about 2600 lbs so I'd look for at least that. I'm guessing you can probably get 2 with new U bolts for around $500 and it sounds like you guys are reasonably mechanically inclined so installing them would be simple enough for you. A whole lot cheaper than a new truck.
Steering not returning to center is usually worn suspension/steering parts, but you've replaced all of those. If the alignment were causing it, I think you would notice that even though you had an alignment done too.