Installing NEMA 14-50 by use of NM-B in a Metal Flex conduit (dry location) code compliant?

Goal: Replace existing 10-30 (8/3 Al on 30A breaker) receptacle with a 14-50 (6/3 Cu on 50A breaker). Receptacle will be used to charge an EV (at 32A, but I think its only right to actually make sure the 14-50 is capable of doing what its supposed to, who knows what next owner of the house may use it for).

Reasons I want to avoid using THHN, location is a 2 car garage, the run starts in wall (behind drywall, about 10 feet), run ends in a wall (behind drywall about 20 feet). The space between these two walls is an existing 3/4in metal flex conduit (length is width of 2 car garage). To use THHN Id have to rip up two walls to install conduit. So NM-B is much more convenient for me. I have to use some sort of conduit (i think) because the ceiling of the garage is unfinished (exposed beams), so there is no drywall or anything to put the NM-B behind.

The conduit is existing (its what the 8/3 Al on 30A breaker is running to the existing NEMA 10-30 receptacle).

Now the questions. Please correct me if I am reading something incorrectly.

From reading NEC it appears that NM-B is not allowed in wet locations. Conduits outdoors are considered wet locations. My entire run is in a dry location, all indoors and more than 8ft off the ground.

Two other concerns that may come up.

1. Derating – being NM-B it is automatically derated in NEC table to 60C and 50A (individual conductors are rated 90C but dertaed as a whole to 60C). So since I am putting it on a 50A breaker and using 14-50, it seems like derating is a moot debate (since its already done automatically).
2. Fill table calculation. NEC specifically says to treat NM-B as one conductor when calculating fill table. I measured my NM-B (it is round not oval) and it is 0.5in diameter. I used Southwire’s fill calculator – https://www.southwire.com/calculator-conduit with the following settings:

• Conduit – Flex FMC 3/4
• Conductors – 0.5in x 1

Calculation gives 37% fill rate. Within NEC spec.

So, what am I missing? Based on this it should be to code for me to run NM-B 6/3 through a 3/4in metal Flex conduit across the ceiling from one wall to another?

Thanks for any feedback!