DD (8) does riding, music and drama/dance/singing.
The drama/dance/singing is via one big, national provider. Tbh I think it's overly expensive for what is actually provided, but it's been invaluable for developing confidence etc.
The music lessons are because she particularly wanted to learn the instrument but also because I've got a generally vague, fuzzy belief that learning music has benefits for young developing brains........probably read some pseudo-science somewhere and believed it!
Riding is a passion and something she seems to have a natural affinity for. I wouldn't be surprised if at some point drama et al are dropped in favour of riding/helping at the stables.
These total approx £255 per month, excluding any petrol, uniform, equipment etc.
DD goes to an independent school where music, LAMBDA, languages etc are available as additional activities - but they are expensive and squashed into lunchtimes when she would rather be playing with her friends.
OP, I think I might be a bit similar to you in that I was also a shy child. I didn't have the chance to do these types of activities - they were simply unaffordable. I feel lucky that I can provide DD with the opportunity to try things, find out what she loves to do or is good at and build life skills.
DD loves down time, spent days reading and playing board games over christmas, but is looking forward to getting back to activities this weekend.
I know of some children who have an activity each evening after school and then a packed weekend schedule. To me this is too much , but everyone's different.
If you can afford the activities and everyone is happy then YANBU. If it gets to a point where it's having a detrimental effect on family time / finances or relationships then you can rethink.
Activities are nice to have, if the child is the sort who thrives on them, but not the be all and end all. I seemed to spend most of my childhood weekends watching TV and being dragged to Sunday school but I turned out alright (I think!) 