a straight-up sincere question

Yes that's what I say

I used to follow everyone first. But then very few followed me back.

Yes, it is polite. There is nothing wrong in it.

It's not so much rude as generally unhelpful. Most people that are likely to do so will have already after you follow them. Those that don't likely would be less likely. I don't follow many, but I go into the galleries of everyone that follows me. If they're generous with their likes, don't publish large amounts every day, respond to people, and have open prompts, I'll follow them. I've never followed anyone because they asked me to.

Just my thoughts on it, though. Others may be different. Sincerely complimenting them on their galleries would likely be more effective, though. I do make exceptions on some of the above for those that actively engage with me. Although the most effective way to gain followers is by doing well in community challenges. Sometimes easier said than done, but it's where most of my followers came from.

Excellent

AND