Your water level is perfect. A few bubbles in lint basket is normal. They usually get sucked in through the lid. More grease might help that, just depends on the lid. Some of the older Hayward baskets warp and never seal 100%.

Unless you see the basket half full of air and making gurgling noises I don't think air is the problem.

Have you tried running the filter in circulate mode {if you have one}? This will let you know if the pressure problem is in the sand tank.

It is starting to sound like the water is not flowing through the sand properly.