I'm not sure if this charger was OK before as I never had one, but I noticed the duty cycle in the ad is "5 SEC on 4 Minutes off". That would give me second thoughts.
I recently beefed up a 10/2 Schumacher, originally with button diodes, similar to what you described. I used a pair of 35 amp studs, drilling the aluminium plate, and soldered the other connections. I think they will handle the transformer. I didn't install a fan as it lasted about 10 years as designed. It had a lot of use.
As for spending 2 to 3 hours on a $29 charger, well retirement is great. Yes, I know I'm slow but half of it was looking through my junk for the diodes. And yes, I know they cost more these days.
Joe and Evelyn