Registers only blowing 90-100 degrees from natural gas forced hot air?

Hi all. So I bought a house this year. It’s a 1200 sq ft ranch with a full basement with 8 foot ceilings. It’s natural gas and it’s a a 4 year old rheem furnace. I believe it’s a 118k btu which should sufficiently be heating up this house.

My registers only seem to be blowing at a max of 100 degrees. Some are only at 90.

My unit is in the basement, and in this house for some reason they run the ducts all the way up into the attic, then the flex ducts run to the rooms from the ceilings and low on the wall as well. The main trunk in the attic is insulated. Each bedroom has 2 registers, one in the ceiling and one low on the wall close to the floor.

I don’t know why they didn’t just do floor registers everywhere straight from the attic. It seems like a lot of ductwork to fill going up to the attic. Also when it starts you can feel it cold at first, and then it warms up. Even still the most I’m seeing is 90 to maybe 100 degrees on certain trunks. The attic is obviously freezing as it’s 9 degrees out today.

The previous owner added an additional trunk in the basement, with 3 registers off of it. On a separate thermostat. There is a damper for the basement trunk, and a damper above the unit, for the main section of the house.

I don’t keep the heat very high. No more than 66 and it has no problem heating up. If I have company I can bump it to 68-70 and it will heat up no problem but I thought I should be getting more like 120 out of the registers?

Since heat rises, am I supposed to be closing the registers on the ceilings?

Should I run the system and go on the attic and check my flex ducts for any leaks?