How far should home A/C be from the setpoint?

Depends how long it’s been sitting on, can’t really answer your question

When it's this hot I would say that is pretty good. Normally it should get to the setpoint.

It shouldn’t be that big of a difference. The set point for when the unit kicks on and turns off is usually not the sam but a four degree difference means something is not right. If your unit is constantly running and not bringing the temp down to set point then you either have a problem or your unit just can’t keep up

Leaky evaporator coil or leak in line

I never had a problem with the A/C unit, but it seem like when I changed my old thermostat to a new Sensi smart thermostat, I’ve had temp offsets.

Also, the unit has been running more since I installed the sensi

The difference between set point and actual temperature is really irrelevant. The industry norm is going to be 25 degrees less than outside temperature as someone already stated. But there are many variables in homes. If the house feels comfortable your thermostat could just need to be recalibrated.

Sounds like the unit isn’t keeping up. Could be a leak or it just needs a good cleaning.
I experienced this a few years ago in the middle of summer, and both of my units just needed to be cleaned.

Is it blowing cold air or is the fan just blowing. Check if the compressor outside is running.

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I never had a problem with the A/C unit, but it seem like when I changed my old thermostat to a new Sensi smart thermostat, I’ve had temp offsets.

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Is there a noticeable change in comfort and do you have a verifiable second way to check temperature?

Typical home AC units may have a tough time reaching 68 at this time of year in the South, ESPECIALLY if the house is older, poorly insulated, etc. It's possible that your AC was never capable of actually reaching 68, but the old thermostat read a little low, so it thought it was 68 when it was really a little warmer in the house and would send the stop signal to the system. If the new one is reading the temperature accurately and the system was never really capable of reaching 68 in the first place, it will run non-stop with the newer thermostat installed and set to 68.

That's why I asked if there was a noticeable difference in comfort. If it's still working like it was before to cool the space, but just is not getting the stop signal, it should feel about the same, but run all the time. Just bumping the thermostat up until it stops running periodically will be all you need to do. After all, the number on the thermostat is completely arbitrary. What's important is whether you're comfortable or not. If the system really is physically degrading, then you'd expect to progressively feel a reduction in comfort as the system gets worse and it'll run all the time.

A second RELIABLE way to check the temperature will verify whether or not the thermostat is correctly measuring the temperature, though it most probably is. Whether you're comfortable or not when you set the thermostat to a temperature that makes the system cycle normally is the real test.

In the five or so homes I have lived in, getting below 70 in the summer has always been a challenge. My current new system will drop the temp from 78 to 71 in less than 30 minutes but from 71 to 68 would take hours during the day.

I also have a sensi and it works great. I think your old thermostat may have been a couple of degrees off compared to the sensi and you are just trying to get the temp down too low.