Free: Contests & Raffles.
To me bases on the location of the crack I would just weld it up as it is behind the axle. However it looks like the leaf springs are to long and not sitting in the right spot. It looks to me like the leaf would hit the frame on bumps and is probably the cause of the crack to begin with. I would look for a shorter leaf then weld the frame or just get a new trailer if cost of fixing is to much. Just my opinion based on the pictures you have.
If you look the front of the leaf is in the bracket but the back is not in the bracket and requires the link to lengthen the point for it to hook up. It looks to me like the original spot was where the link is hooked to but springs may have been swapped and the link put in to fit the longer leaf. I could be completely wrong. I will have to look at my trailer when I get home and see how it is set up.
I was taught to never buy a painted boat trailer. Now I see why! Sorry for your misfortune
https://seattle.craigslist.org/skc/bpo/d/auburn-galvanized-ez-loader-boat-trailer/7770592515.html
How far is your commute to the launch? I would also take a hammer and tap the frame to see if there are more areas with internal corrosion and even run a borescope or drain scope in there to take a closer look. I broke a trailer in half on the way to Sekiu one year and there was no external sign of problems. It was just rusted form the inside out.