Recurring shower odor with slow drainage: clog, trap, or venting?

A shower drain has developed a recurring musty or sewer-like odor, and the water now drains noticeably slower than it used to. The shower is used regularly, so a completely dry P-trap seems less likely, but the smell is strongest after the shower has dried rather than while water is running.

There isn’t standing water for long, although a shallow pool forms around the drain during use. Before assuming the trap is clogged, I’m trying to separate a hair or soap buildup issue from a venting or drain-slope problem.

A few diagnostic questions:

- If the odor improves temporarily after flushing the drain with water, does that point more toward trap evaporation or buildup?
- Could a partially blocked vent cause slow drainage and odor even when the drain itself isn’t fully clogged?
- Would an improperly sloped horizontal drain typically leave water pooling, or would it more often cause recurring odor?
- Should nearby fixtures gurgling when the shower drains be treated as a strong sign of a venting problem?

The main thing I’m unsure about is whether to start with a careful drain and trap inspection, or have a plumber check the vent and drain slope before using a chemical cleaner.