ERD directly producing 5NF relational schema - is this desirable?
If your tables are already in 5NF then you don't need to think much more about further normalization. 

Note however that the principle of full normalization (POFN) is no panacea. There may remain non-obvious update anomalies. 

Database constraints will need to be applied. In my experience, designers who start with an ER diagram are great at implementing primary keys and foreign keys but other constraints can be easily overlooked. Other design approaches may result in better constraints e.g. I have a little experience of object role modelling which seemed to work well.

Further normalization should only be considered in conjunction with constraints because 6NF often makes constraints more difficult to implement using today's industrial-strength technologies e.g. none support Standard SQL's CREATE ASSERTION nor CHECK constraints that may include a subquery. 

Also consider the principle of orthogonal design (POOD).