is growing followers on instagram still worth the effort?

for a lot of small businesses, honestly no, not as a goal on its own. the count mostly matters for looking credible when someone checks you out before buying. past a few hundred real ones, put the effort into getting the profile to send people somewhere - the link, a dm, a booking.

Depends entirely on what you want it to do. a big cold following converts worse than a small one that actually replies to you. if you're selling locally, the number that matters is dms and saves, not followers. what are you trying to get out of it?