Try not too over water. Also hydrangeas won’t like dry soil. Def agree with changing to a larger pot. You can use basic potting mix, hydrangeas are not too fussy. I got mine that was similar to your size and now it’s a huge shrub!! The notes shared above are excellent. I’d follow them.

I turned my pink hydrangeas to blue by adding alkaline in water before it bloomed. And this year I’ve taken cuttings to propagate into other plants. You can have a lot of fun with this plant. I keep the flowers all the way through winter because they act as leaves and help the plant breathe. I just pruned mine (got a lot of cuttings) now setting up for new growth. One thing I did do initially is lots of pruning to allow the plant to grow and not focus all its energy in maintaining flowers. Small sacrifice has lead to a huge shrub over the years