OP, nice choice!
50k and 4+ years old.
1. Change oil by mileage unless you can verify that it's been changed at 6 months previously. Reason, 6mo intervals are totally unnecessary in and of themselves , save for possible warranty coverage issues from not "following" the recommendation. You'll be out of warranty soon anyway, if not already based on the 5 year mark.
Save your money for useful maintenance.
2. No "additives" needed for anything. Fuel and DEF are the only 2 things you need to put in the tanks.
3. With that mileage and age, I'd service the trans, complete fluid exchange, diffs and transfer case and start thinking bout coolant/brake fluid and hydro boost fluid change in the next year.
4. Fuel filters, easy to DIY. Make sure you're buying the right quality/micron rating and not knock off filters.
5. That is all. Enjoy!
