Is it worth getting a computer science degree after you are already in a senior role?

Or... should I just complete an open courseware course to fill in some of the gaps?

If you already have more than one 2 years industry experience you don't really need a degree. The only exception is if you want to work for a really big conglomerate. They're more likely to force you to go through an HR gauntlet first, which is more likely to automatically fail you for not having a degree.

Pretty much anywhere else doesn't care, as long as you have a proven track record.

Good question! 😃
You should do something at least. As you get older without certificates it gets really hard if you have to search for a new job. Employers usually don't give a damm about workforce 50y and up without papers that show they're interested and up to date.