New furnace trips the high limit switch - is my ductwork the problem?

Went thru almost the same thing after we downsized our furnace a few years back. Same pattern, runs a while, trips the limit, comes back on. The fix turned out to be two cheap things before anyone touched ductwork. First thing to do is check the temprature rise. Get a cheap probe thermometer, stick it in the supply plenum and one in the return near the furnace, and compare the difference to the range printed on the furnace rating plate. If your rise is way above the high end of that range, the furnace is making heat faster than the blower can carry it away, which is exactly what trips a limit. That measurement tells you for sure wether it's an airflow problem or something else, and it takes ten minutes.

If the rise is too high, the next thing is the blower speed. A lot of furnaces ship on a lower tap from the factory, and your installer may never have moved it. Bumping the blower up a speed is free and often solves this exact problem on its own. Have your tech check the manifold gas pressure too, because if it's set high the furnace is putting out more heat than the plate says, and that alone can cause constant tripping. On the door question, yes, a bigger opening can genuinely help, because a louvered door in a small utility room is a real bottleneck. But the louver area matters more than people think, and taking the louvers out is not the same as having a proper opening. If the return trunk itself is undersized, no amount of door opening fixes it, which is why the temp rise check matters. It tells you whether you gained enough from the door change or whether the restriction is deeper in the ducts. Do the thermometer test and the blower tap before spending anything on ductwork. In my case the higher fan speed alone dropped the rise into spec and the cycling stopped completly.

Something else to check before blaming the trunk. When mine did this after a downsize I grabbed one of those cheap camera attachments for my phone and snaked it into the return drop near the furnace. Turned out a chunk of the old flex duct lining had come loose and was half blocking the passage. Would have never guessed it from outside. Took ten minutes to pull the blower door and shine a light up there first, camera confirmed the rest. Worth a look if your returns are old.

I'd pump the brakes on the blower speed advice. On a new Rheem there's a good chance the blower isn't a belt or a tap you flip, it's an ECM motor and the board decides airflow off dip switches or a commissioning setting. Telling someone to just bump it a speed assumes an old school multi tap blower, and if they start flipping stuff without knowing what it does they can end up worse off. Have the installing tech set the airflow to match the temperature rise, that's literally part of commissioning a new furnace and they should come back and do it for free if it was missed.

One thing I'd add since everyone's focused on the ducts. When my old unit got swapped out a few years back the installer reused the existing AC coil sitting right on top of the new furnace, and nobody looked at it. If yours has a coil from the previous setup, that thing has been collecting dust and lint for years, and it's a way bigger restriction than any return duct. A dirty coil does the exact same thing, heats build up, limit trips, recovers, repeats. Ask the tech if the coil was inspected or replaced during the changeout, because alot of installs just drop the new furnace in under the old one and call it done.