Subaru Forester Catalytic Converter

I have a 2014 Subaru Forester that from
almost day one burned oil like crazy. Was told
for many visits it was the extra cold weather
that year, or the lots of kms, or even the PCV
valve. Then got the warranty letter from
Subaru, so received my engine replacement in
2018. Since then I’ve had the engine light go
off sporadically, and doesn’t always stay on.
At first thought it was gas cap, but tightening
that no longer worked. Then got the
diagnostics done. P0420 code - was quoted
for catalytic converter and sensors. Left it for
awhile as it wasn’t affecting performance of
vehicle but damn it’s annoying to not use
cruise control when the engine light decides
to turn on. Then I got a second opinion (not
the dealership) who tried a quick fix by adding
a small part to try and trick the sensors, and
suggested I push things further with Subaru
as it was likely the oil consumption issue that
damaged the sensors and cat. Before I go in
there and state my case, I’m wondering if this
all makes sense, and if there are other Subaru
owners going through the same thing or have
any success stories with getting it repaired?
Thanks!