As Radiohead said, the masthead amp should be on the mast close to the antenna, to allow max signal quality into it.
Being in the roof will reduce the life, but not to the extent yours has been. If yours truly did fail, (I’m not certain it did, I’ve never seen a failing/failed one give those symptoms, not to say that they don’t).
If yours was only 4 years old, it’s just made it passed the 3 year warranty, like I said before, it’s not uncommon for a Kingray amp to go for 15-30 years.
I’ve replaced 10-15 year old ones literally over the road from the beach due to some corrosion. I think I’ve only ever replaced one about 5 years old & that was because more outlets were added & it wasn’t high enough gain, I refused it on another job, (gave it to the customer for free).
The masthead amps are in a weatherproof sealed box, small amounts of air & moisture get in, but it’s so small amount, it takes a long time to start corroding things & effect it’s operation.
Admittedly these are not in north Queensland, but i’ve put masthead amps in roofs before & to my knowledge they’re still going more than 5 years later.
Splitters of the old screw & saddle type design, installed under houses, give far more corrosion problems, than masthead amps.
I’d be interested to know the signal strengths at the antenna & outlets, (before & now), what was the old masthead amp model, how high or low was the adjustable gain set (if it had it), & how high or low is the adjustable gain set on the new one, they’re questions I’d like answers to, but not something you can answer unfortunately.
New power supply needed, quite possibly, they do fail in short periods of time, (I’ve had at least one fail within a 6 week period of being installed). I had one at my old house that was 11 years old & still going strong when I left though.
New masthead amp, I’m still not sure was needed, but honestly can’t say from here, without knowing more information.