Yes, of course, I should have taken note of the name, H. 'blue bird' is definitely a H. syriacus, outdoors, then.
It's a vigorous shrub, so certainly not happy to be grown in a pot for 6 years! You do not show pics of the specimen planted in the ground. I do not know about Aldi since I do not live in the UK but I never buy cheap plants from such places. I much prefer to buy from "real" nurseries to avoid disappointment. The only piece of advice I can give for 6 year old hisbiscus specimens which have never flowered is to throw them away and buy replacements from a reputable nursery.
Posts
When you don't even know who's in the team
S.Yorkshire/Derbyshire border