Help With Troublesome System

[Greatwhitewing]

Existing house second floor approx 1000 sf and similar for first floor, oil fired Viessmann boiler using baseboard hydronic heating. 4 zones, one for finished basement, one for first floor, one for domestic hot water, one WAS for second floor.

Added a 550 sf in-law apartment over an open deck (second floor level attached to house) using 2'nd floor hydronic circuit for a hydro-air to heat while adding AC for apartment and second floor. Added two 6" drops to first floor with marginal but improved comfort downstairs. All done by by mid level (25 employee) so-called pro who botched the install in almost every way possible. IOW I got screwed

Hydro-air is First Co 36HXHB and AC is 3 ton Gibson with poor SEER. Two zones with Honeywell NC zone valves.

History- had MANY MANY failures early on on this 3 year old install with excessive humidity shorting out about 6 main boards. Finally found ONE contractor who recognized the issue and ADDED more bypass and return vents. It helps but still getting excessive humidity with accumulation of an inch or so of water in overflow pan that has drainage combined to condensate line downstream.

My concern is corrosion of the air handler with the inside sopping wet and a lots of external condensation. Estimated about a gallon per day on hot/humid days of external condensation. I added a small dehumidifier but virtually useless. Ordered a much larger dehumidifier rated 150 pints/day with a pump to keep attic below dewpoint (hopefully). The attic is considered conditioned but it's 85/85 on hot days.

I have done all I can do to insulate air handler externally. All it accomplished was just moving all the condensation to the bottom panel where I cannot access.

Had a reputable contractor come in and offer a top of the line Bryant variable speed heat pump with communicating air handler for a large cost (~$20k). Promised to add returns if needed and seems to back their work with pretty good reviews online. Offered to add an additional 8" drop to areas still warm (kitchen/living room) for an additional $1.1k which would make the downstairs comfy.

Like to hear from pros and others that may have had similar issues.

Heating mode is fine, no issues at all. I am in southern/central MA with a lot of solar exposure.

Looking forward to a constructive discussion.