York HH824E2S11 (2 ton inverter): compressor never uses its full range below ~72F return. Is coil temp capping it, and can I do anything about it?

Setup: HH824E2S11 heat pump (R-454B, 3 weeks old), TM9E060B12 gas furnace as air handler, matched coil with TXV, Ecobee, 1800 sqft brick house in Utah. I'll follow up with results on anything suggested.

Basics already covered:

- Clean MERV 8 filter, return grille clear, all registers open, blower wheel inspected clean

- Blower on MED-HI tap, about 1250 cfm per the furnace tables

- Static measured with a manometer: about 0.2 inwc total including filter, big old ductwork

- Charge verified with gauges: 10.3F subcool, 15.2F superheat, 43.5F evap sat

- High stage input (Y/Y2) confirmed energized at the unit during calls

- Voltage healthy (247V), no fault codes, no frequency limit code (P.18 = 0)

- This unit replaced a previous York inverter that showed similar behavior, so I doubt it's a defective machine. After months of struggling with the last unit, got it replaced under warranty with a similar heat pump. The fact that it's doing similar behavior as the previous one suggests to me that the issue is inside the house/furnace/ducts, not with the unit itself.

What I see on the outdoor unit's parameter display: the compressor lives at 40 to 50 Hz and never goes higher outside of forced service mode (which proves it can). It tracks return air temp. House at 74+: suction temp (P.6) runs 10-11C and it works harder. House pulled down to 68-69: suction temp slides to 5-6C and the target frequency (P.4) and demand (P.11) both walk DOWN while the thermostat is still calling. Looks like maybe it derates itself off the cold coil with no error/limit codes.

Result: holds 72-73 all day at 99F outside with margin, but pulling down from 72 to 68 in the evening (don't judge, my partner needs it cold to sleep) takes 9+ hours of continuous running I think because capacity shrinks as the return cools and it never goes to full effort. I believe that the old unit was previously able to cool the house much faster than this before issues started in June. Now both old and new units can't perform like I remember the old one doing in the past.

Dealer and York have both been involved along the way (hence the warranty swap), so this isn't in place of professional service, I'm just trying to understand the platform's control behavior.

Questions:

What could have gone wrong inside the house to cause the heat pump to degrade in efficacy, and limit it's compression effort?

Is this size of derate normal for this platform? It acts like capacity roughly halves between a 72F and 68F return. Or is mine unusually conservative?

What is the control actually watching, and is there any dip switch or service setting that adjusts the floor?

Any legitimate way to get full effort for a deliberate deep pulldown (pre cooling at night), or is this just how these inverters are and I should work with it?

I know 68 setpoints in August ask a lot from a 2 ton (though shouldn't be so bad at night). Just want to know if this is spec or config. Can post more parameter readings, I've been logging everything in Home Assistant so I have a lot (maybe too much) data.