I was wondering, I flew about 1,100 miles one way then back (no stops). The first path we were very high up I'd assume like 30K feet, the way back we flew I'd say 10K feet, much lower. I was wondering why.

Possibly to do with how the jet streams were at that time.

For longer flights, large passenger aircraft will generally aim for something like 30k feet - the thinner air means less air resistance so more fuel efficiency (the cost is the fuel required to get there, which is why I said "for longer flights").

However, at that altitude the "jet stream" can cause winds of 100+ mph. This can be helpful with your direction of travel, or it can slow you down. If the jet stream was strong and trying to blow you backwards, it may have been more efficient to travel at a lower altitude where the jet stream wasn't present.

For a long & low trip like that, ATC staffing is a common story. Higher sectors not having the manpower leads to clearance delays, and this is a somewhat common workaround for that somewhat rare situation.

If this was a jet there’s no good technical reason for it to stay so low, but one common reason could be air traffic control congestion, you go through different sectors at 10k than you would at 30k so it’s sometimes feasible to go lower and avoid long delays.

There are many factors that could have contributed to this (airspace restrictions, turbulence, etc...), but usually altitudes are selected based on the prevailing winds. You want as strong of a tailwind, or at least as weak of a headwind as possible.