there's an addon, called the Flex-shaft, for the Dremel that lets you use it like a pen, fucking love that tool

Don’t forget that an orbital sander with a thick foam pad between the base and sanding paper also does a good job of rounded surfaces. You can also get drill/dremel bits that provide similar functionality for smaller hook and loop discs. Eg. https://www.amazon.com.au/HEATSHAKING-Pneumatic-Attachment-Industrial-Polishing/dp/B0G499XZLX

I find this lasts a bit longer and more versatile than a flap wheel.

There’s also the super diy approach by making a custom shaft to hold standard sand paper which works well in a pinch https://www.instructables.com/Simple-DIY-Flap-Sander/

I have a big bag of knock offs from Amazon and can confirm that they work well!

I ended up using those to sand some finger grooves in cutting boards. They're surprisingly aggressive.