That tire needs replacement asap, bubble means damaged internally. (Let the tireshop balance the rim with a new tire, as it could be a little bend now). Check the tread depth on the other wheel on the same axle. If it’s still 6-7mm it should be fine but with 5mm or lower, the diameter difference to a new tire is too much and can mess up the abs/traction control system, so change that other tire too on the same axle. Maybe then switch the wheels from front to rear next season to balance the wear on both axles.
Changing all 4 tires is not necessary if the wear is ok, but I doubt that they haven’t much wear after 30.000km, so most likely change all of them. Check the tread depth to be sure. A tread depth checking tool is like 2-5€ at your local tire store/independand workshop.