Note: I have very little understanding of Greek grammar. Please take this answer with a grain of salt.
The NET Bible notes on verse 5:
By predestining. The aorist participle may be translated either causally (“because he predestined,” “having predestined”) or instrumentally (“by predestining”). A causal nuance would suggest that God’s predestination of certain individuals prompted his choice of them. An instrumental nuance would suggest that the means by which God’s choice was accomplished was by predestination. The instrumental view is somewhat more likely in light of normal Greek syntax (i.e., an aorist participle following an aorist main verb is more likely to be instrumental than causal).
Using the instrumental sense, the NET Bible renders verses 5-6:
He did this by predestining us to adoption as his sons through Jesus Christ, according to the pleasure of his will—to the praise of the glory of his grace that he has freely bestowed on us in his dearly loved Son.
In other words, Paul suggests that God's choosing of some people for adoption was a more complicated process than simply picking their names. The process seems to be:
- God decided to bestow His glory on Jesus Christ.
- He decided to adopt some people to be His children.1
- He picked the people he would chose "before the foundation of the world". (v. 4)
- He "blessed us with every spiritual blessing". (v. 3)
Therefore, not only was it God's will that certain people be predestined, but it was also His will that predestination be the mechanism for glorifying Jesus. What might seem like a capricious act (like preferring one flavor of ice cream over another) is actually a carefully considered decision with a definite aim.
Verse 11 is similar:
In Christ we too have been claimed as God’s own possession, since we were predestined according to the one purpose of him who accomplishes all things according to the counsel of his will so that we, who were the first to set our hope on Christ, would be to the praise of his glory.—Ephesians 1:11-12 (NET)
If βουλή is translated as "counsel" it evokes the idea of God deliberating with Himself (or among the Trinity) and coming up with a course of action. The broad plan was to use predestination and as a result God claimed some as His possession.
Repetition is a useful technique for implanting a specific idea in an audience. In this case, Paul repeats with various words the idea that God's will trumps ours. The whole letter turns on this theme:
For by grace you are saved through faith, and this is not from yourselves, it is the gift of God; it is not from works, so that no one can boast.—Ephesians 2:8-9 (NET)
Ephesians 1:3-14 might be an example of a berakah psalm.2 If so, the repetition and rephrasing of the theme would have a literary purpose within the genre. This would be true even if God's "will", "pleasure", and "plan" referred to precisely the same concept.
Even so, I think Paul is explaining that God's plan was rather intricate (why send Jesus to suffer and die if God could have just picked some people for adoption?) and that the compilation was no accident but intentional.
This is a particular type of adoption as the NET note explains:
The Greek term υἱοθεσία (Juioqesia) was originally a legal technical term for adoption as a son with full rights of inheritance. BDAG 1024 s.v. notes, “a legal t.t. of ‘adoption’ of children, in our lit., i.e. in Paul, only in a transferred sense of a transcendent filial relationship between God and humans (with the legal aspect, not gender specificity, as major semantic component).”
Or a hymn of praise. Roughly 1/3 of the Psalms are of this type.