I wonder if he had that idea before or after Patrick Stewart was cast as Picard.

After/during the casting process. Originally, Gene Roddenberry didn't want a bald actor to play Picard, becausr in his opinion there should be a cure to male pattern baldness in the future. Stewart was even asked to bring a hair piece for the audition. However, Roddenberry was talked out of it by basically everyone else with any responsibility in the casting process (partly because Stewart looked somewhat cursed with a hairpiece).

The thing is, by this point in their timeline, hair loss isn't a stigma nor considered a deficiency.

In my head canon society had developed such that male pattern baldness was seen as natural and not anything that needed to be cured. But the Picard series sort of debunked that

Funny enough, I'm good friends with the woman who was the lead researcher of Proctor and Gambles' program for a hair growth product back in the 90's. Ultimately, P&G was forced to abandon the effort after many many years.

My friend stated she could grow hair on a cue ball. The only issue was the cue ball wouldn't survive the process.

Short story long... It is possible to cure male and female baldness. The only problem is the patient would not survive the toxic cocktail that growing hair requires.

Picard's baldness is a choice, just like Geordi's visor.

Edit: And Kovich's glasses

When Seven was first rescued, the Doctor talked about stimulating her hair folicals. He called a vicarious experience.

So clearly there is a way to "cure ' it, if someone did want to.

Not everyone cares about being bold. Plus he knows Guinan likes it.

There's nothing wrong with being bald

aside from the others, here's Patrick Stewart talking about baldness and hairpieces : https://youtu.be/bqm_Iq8rFeg?t=93

"In the future, nobody will care"

Rodenberry was already asked this question. That was his answer.

I wonder if he had that idea before or after Patrick Stewart was cast as Picard.

Patrick Stewart showed up wearing his customary toupee. It looked so ridiculous that they asked him to lose it and just go with a bald character.

EDIT:

https://youtu.be/PTu1FX1rOFQ

Star Trek in many ways is completely off the mark wrt to the levels of technology portrayed.

https://youtu.be/zklIGXVdibo

He he is playing Hamlet circa 1969. Looks like a wig to me, but what do I know?