Heat only on max

That's because the air hasn't been bled out yet. Once you have the gurgling in the heat open the radiator to the first detent let the pressure blow off then take the cap off let it sit for about an hour or a little better and see where you are on your coolant after you do that and it's cool fill the radiator slap up fill the jug a little over the full line then get it hot again do the same thing now it should be getting stable and if your jug is a little full you can suck the water out with a turkey baster to the full mark or leave it if it's just say a half inch or so and that should do it.

 

If you are getting, literally, strong heat when you have the desired temperature set at HI and it flips immediately to no heat when you click down to the first numbered-degrees temperature setting below HI ... that behavior is nearly always related to the cabin temperature sensor inside the car. You can read more about that here. There is no easy story connecting behavior like that to a problem with coolant fill, as a problem there would also be apparent on the HI setting.

However, maybe the situation you meant to describe wasn't really, literally, strong heat at HI and no heat at the next numbered setting down? If those aren't the symptoms, the cause could well be something else. What puzzles me about the indoor cabin temperature sensor is that it doesn't sound like any work was done in that vicinity. (The sensor is in a dash panel kinda below the power switch, above the driver's right knee.)

 

If you hear gurgles, then you probably still have air in the system. I would run the engine up to full operating temperature while monitoring ECT sensor with a scantool. Then go on a drive, lots of heavy throttle to get the RPMs up. Recheck coolant level after it cools down.

Eliminate the known problems then see what's left. Anything further would need a capable scantool to check A/C data.

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I agree with what others are saying, air is still in the system. The air bleed procedure is not complete, that's why I use the vacuum method to refill the coolant if I want to do it quick (in one shot)
Here's how I bleed the system the easy way (without any tools) but it will take a long while. With the engine cold (don't turn on), open the radiator cap and fill it up with coolant. If the coolant is still bubbling while filling it up to the fullest level then there's still air. Continue filling until it will not accept any more coolant and close the cap. Turn on the car, set your temp to max and blower to max in the cabin, wait for the engine to run and let it run for a while until it turns off automatically. Turn off the car and open the radiator cap and fill it up again, repeat this process until you see no bubbles and coolant will not suck in anymore. Drive the car for 10mins around the block (or until operating temp) and park it, turn off and let it cool down.. check the coolant level by opening the radiator cap and fill it up again with coolant, repeat the process until no bubbles show up and will not suck in more coolant. Don't be afraid to overfill it on the radiator neck, it will just spill into the coolant reservoir tank.
You can also do the bleeder valve method on the driver side of the radiator, that's also faster compared to the method above.

 

The thermostat is on the engine, operates only based on coolant temperature, and doesn't make sounds, so that's not what you're hearing open up.

When you select max heat and you hear something move and the air is immediately hot, what you are hearing is the air mix door.

The air mix door controls whether the air blowing through your ducts goes through the heater core or not. Because the coolant always circulates through the core and the coolant is already hot, the core is also already hot, and that's why the instant the mix door moves to send your air through it, the air gets immediately hot.

So the question to answer here is: why does the air mix door move to the heating position only when you select max heat, and move back to the no-heating position the moment you go one notch down?

The answer to that question is nearly always going to be as described in this post.