Depends on what you want to achieve - professional or personal growth. If you want to use the certificate for getting a job, depends on how many companies value the course. I took a Tai chi for health course online - given that there were no similar instructors / courses locally available, I felt it was worth the money.

I have been looking at learning portrait making online.

https://vitruvianstudio.com

Vitruvian studios course seems to have good feedback overall.

Not sure entirely as I have not taken it yet. But the reviews seem pretty good.