I’ve been where you are and would echo what oaklandopen said. I previously owned a 2017 2500 Limited with the Cummins heavily optioned that only had 1900 lbs cargo cap due to the weight of the diesel and tranny. By contrast I now have a 2019 2500 Laramie with the HEMI also pretty well optioned that has over a 1,000 lbs more cargo capability and slightly less than 3,000 lbs towing. Given I’m towing, a 11k lb travel trailer with 3 slides, approx 1300 lb tongue weight, and the number of times and distance I tow, the big HEMI was the best fit for the money. The HEMI is significantly cheaper and doesn’t have the additional maintenance requirements. They are different things and both definitely have their place for the application you intend to use it for. I would advise if you want a diesel then go 3500, if not, a 2500 with the HEMI is nearly as capable and lots cheaper, especially over the long term.
I’ve added a lot of aftermarket stuff to all the vehicles I’ve owned over the years, some worked well and some didn’t. I am of the opinion that factory installed options tend to work better and integrate more seamlessly with the overall systems design. Honestly I don’t think I saved any money by going aftermarket other than not incurring interest on the item in the loan. I added aftermarket air bags with an onboard compressor to my current truck to help distribute the tongue weight. I think all in it cost me a little over $800 installing it myself. It would’ve been around $1400 - $1500 at a specialty shop. The factory option of rear leveling suspension is around $1500 - $1600. The difference is with the Air Lift system I have input and some degree of control BUT... have to learn where the sweet spots are and it’s only my subjective judgment if it’s right or not. The factory option is SELF LEVELING which I had in my previous truck and I loved it because it ran on magic, it just worked and I didn’t need to mess with it. I hitched up the travel trailer, it would squat then the compressor kicked and ta-da the old girl came back to level all on its own.
Good luck with your decision, you’ll be very happy with whatever you go with. These trucks are more capable with each new year model.