I am assuming here that it is running in burn gas mode, not as a heat pump. According to the nameplate, the unit has an output of 56,700 BTU per hour and is designed for an output delta-T of 30-60F with a maximum discharge temperature of 160F. The symptom of the overheat breaker tripping suggests that the maximum discharge temperature is being exceeded. Assuming that it isn't a failing sensor / breaker and that it really is getting too hot, the output air temperature is going to be a function of the BTU per hour and the air flow. In turn, the air flow is going to be a function of duct size.
I seem to have misplaced by air flow / versus duct size tool, so I will assume these figures are correct:
http://library.coburns.com/hvactechtips/Charts/Coburns Duct Size Chart.pdf According to this, a 12" flex duct will supply about 430 CFM. Next according to this calculator(
https://www.coburns.com/hvac-calculators/CFM-with-Gas-Heat), the air flow with a 56,700 BTU heat source to provide a 60F rise is 875 CFM and a 30F rise will require 1750 CFM. For comparison purposes, here's another typical duct size versus CFM
chart Of course the air velocity is another aspect to consider, which is CFM (ft^3 )divided by the orifice diameter (ft^2) and you need to make sure that this is within the acceptable range for the material, least you get excessive loss.
Bottom line, it sounds to me like the supply duct is too small. Interestingly, I read a blurb about 12" being standard for mobile home installation and non-mobile home tends to go bigger.