Spring hanger torn off...probably my fault!

After working two 70-hour weeks, motivated to do something I wanted to do for a change (and the extra cash to do it with!), I thought this was the perfect weekend to start my "axle replacement" final push, before my next outing on April 25 (I've been planning either strengthening it, or replacing it entirely with a heavier-duty version). As usual, I researched alternatives, and shopped local sources for possibilities before jumping in. However, after finding a suitable replacement axle- I had finally decided not to try to sleeve it, since I'm a beginner welder. I decided on a Carry-on 2k axle w/hubs at Tractor Supply. So, in last-minute double-check, before going down to purchase it, I went to the garage for some FINAL, ACCURATE measurements. I crawled under, and scraped away some undercoating/paint from the axle tube -it is only 1.25" square, as I feared, very spindly for a 1515lb trailer- and I confirmed that the TSC axle would fit. Then I went inside to retrieve my phone/camera to take a "before" picture, prior to the swap. When I returned from the house, I came face to face with this, somehow overlooked before: I may have only found this break (the hanger welded to the frame, holding the right rear shackle) because I had re-levelled the trailer a few minutes before, shifting the weight. I had not seen it in the 5 months since I had camped in it, though I had been under it several times since then, including 20 minutes immediately prior. The torn piece has shiny, un-rusted edges, so I must've had it break just then. Fortunate that it happened in the garage! I'll never know if it was caused by my ill-advised "axle safety" attachments, that probably forced the metal fatigue at that location (by reducing the ability of the springs to flex and absorb energy?, since I clamped the leaves tightly together-good for dragracing launches, not so good for trailer springs), or it was inevitable with the oversized load on an undersized, old, basic frame. I now have to patch, and reinforce, the torn spot, remove the other (suspectedly weakened) hangers, and start over. The basic frame is made of thinner material than my additional pieces (3"square tube tongue, 12" front and 24" rear platforms of rectangular tubing), so I'll want to weld and bolt-on new hangers. Additional plan: the original frame tubing will have a 3 foot length of 1/4" angle welded/bolted to the inside bottom of the frame. After having balanced my trailer to make it work as it is, I plan on put installing the new axle in the same location, but with new-longer- springs, so I'll get the whole axle package as a kit. I do have a question, though: for my 1515 lb (current loaded weight) or maybe 1600 lb trailer (after new axle, springs, reinforcement), would I be better off getting a 3500 lb axle (might as well get heavier-duty) with 1750-2000 lb springs, or get even heavier rated springs? And did I mention shock absorbers to come?