Take Serbia vs Bulgaria. It’s been 20 years since they became members.

GDP per capita is $20,000 for Bulgaria vs $15,000 for Serbia both of which were at a bit above $6,000 when Bulgaria joined in 2007. That was after being pretty much at the same level for the decade earlier.

Also yes the Union did not expand at the same rate as before 2008. The eastern extensions were absolutely massive. At the same time there has been very significant movement since. Like the Euro was adopted by Latvia, Estonia, Lithuania, Bulgaria, Slovakia and Croatia since then. It is down to six members not having the Euro. There are plenty of other laws which have passed since, which are often massive.