re: New AC running much longer than old one

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Of course, it stays at 69 because it's trying to KEEP it there but why won't it come on only if the temp rises to 70 instead?

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That is a t-stat "swing" or differential setting. Most people dont want a swing as large as you are expecting. Most people want a temperature swing around .5F or 1F worst case. At 1F and 69F setpoint, it should come on at 69.5F and then turn off at 68.5F and probably display "69F" the entire time.

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If it's set at 69 then it will run until it's 69 then stop for 8-15 minutes. Then runs again for 15 minutes even though the temp is still 69 and will stop for 15 minutes. The cycle repeats itself continuously even with the temp staying at 69 degrees. Of course, it stays at 69 because it's trying to KEEP it there but why won't it come on only if the temp rises to 70 instead?

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This setting is called the "thermostat temperature swing". Most thermostats can be set in 1/5 or 1/2 of a degree increments. It sounds like yours is set WAY too low. For example if yours is set to .2 degrees, it will kick on when the thermostat reaches 69.2 and turn off when it hits 69.0. Find your thermostat's temperature swing setting and tell us what it's set to and increase it and report back.

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It runs for 15 minutes and stops for 15 minutes even when the temp is at the set temp.

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As already covered, that behavior is completely normal and expected when a system is maintaining and not near full load. It is having to run that much to maintain the temperature you want. Adjust your t-stat "swing" or "differential" setting if you want to see a larger swing between off time and on time. Drastically oversized systems such as yours will cycle way more than properly sized systems resulting in much shorter life spans than properly sized systems.

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And my 15 year old unit I had before this one legitimately only ran 10, MAYBE 11 hours a day when it was upper 90's to 100 for highs. Now it's nothing for it to run 12 hours a day with highs in the low 90's. This one is supposed to be more efficient than my previous one but it's clearly not.

Either I'm a complete retard when it comes to understanding HVAC systems or this one is a lemon.

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Run time doesn't exactly indicate efficiency. Run time is going to provide a comparison of cooling capacity between units. kWh is how you'd determine if one is more efficient than the other.

I'm not familiar with your previous and current systems but sometimes newer matched systems have lower btuh ratings. For example, 3-tons used to mean 36,000 btuhs MINIMUM and 3-ton systems might actually have more capability than that. Now days, a 3-ton AHRI matched system might be as low as 34,000 btuhs. the new system has less capacity but is more efficient. it will run a bit longer than the old system but still use less kWh. So if an old system was 37,000 btuh rated and new is 34,000. That is a 10% increase in run-time for the newer unit.