I agree with you. One of the reasons for Brazil's poor performance is their coach. 62-year-old Dorival Junior is not an experienced coach. He previously coached Sao Paulo and Flamengo. Brazil is a renowned team. As their goal was to do well in the World Cup and win the Copa, Brazil needed a more experienced coach. Also I saw Brazilian players ignore Coach Dorival. They don't give enough respect. However, this could also be my misunderstanding.
Brazil should definitely try to strengthen the bonding between the players. Because there is no shortage of talented players in Brazil's squad. If the chemistry between the players is good then this team will be strong.
I understand the importance of having a young and vibrant couch especially in a team like Brazil but this can not be justify as one of the reason for their irritating performance. Brazil performance was not only poor, it was very embarrassing and insulting for a team that have produced famous and talented players in the history of football and are also known for taking sports activities very serious to end up playing the way played in a tournament as serious as Euro.
I don't know much about their couch but even if he's less experienced as you pointed out, the players can adjust while in the pitch to make things work right. Majority of their squad players play in a very recognized clubs and have experience to play even without the instruction of a couch. What I could figure out from their overall performance is lack of dedication. They were playing like people who weren't interested but forced to come and participate. The couch should be relief from the duty because at that age, nothing much is expected again.
Personally, I have to agree with what you said, where Brazil current performance has decreased significantly. In almost every line, Brazil has big names, they should be able to perform optimally by giving their best. It's difficult to find out where Brazil shortcomings lie at the moment, in my point of view it probably lies with their manager, and it could also be with the players themselves.
At first glance what Ronaldinho said seems to have some truth, I know it's just advertising, but looking at the current facts, it seems very relevant. The Brazil we expected failed to live up to expectations, they seemed to lose their identity when they played, maybe also because they lost a player like Neymar. Brazil must look for a more qualified manager in the next World Cup event.
On the other hand, Brazilian players should have a respectful attitude towards the manager, in several social media posts, the players can be seen ignoring the coach instructions. Many fans regretted this attitude, it should not have happened in front of cameras and fans. An attitude of mutual trust and respect can make a team solid, Brazil is currently losing this.