If you want to propose a significant difference between boulen and theléma in the biblical use, it would require to search all the NT and see the usage, to decide whether these terms really always differ or whether they are synonymous...
It would be interesting to see.
It would be interesting to see.
[from Bible Hub]
1012 boulḗ – properly, a resolved plan, used particularly of the immutable aspect of God's plan – purposefully arranging all physical circumstances, which guarantees every scene of life works to His eternal purpose.
This level of God's plan (1012 /boulḗ) demonstrates He is the Lord of history, i.e. always in charge!
[1012 (boulḗ) is more than God's immutable plan of physical circumstances. It always also includes the Lord's purpose in them – and hence arranging all the physical scenes of history before creation (Ps 139:16; Jn 1:3).]
^ [con't] "...i.e. (objectively) advice, or (by implication) purpose -- + advise, counsel, will."... "decree"
[...also from Bible Hub]
2307 thélēma (from 2309 /thélō, "to desire, wish") – properly, a desire (wish), often referring to God's "preferred-will," i.e. His "best-offer" to people which can be accepted or rejected.
[Note the -ma suffix, focusing on the result hoped for with the particular desire (wish). 2307 (thélēma) is nearly always used of God, referring to His preferred-will. Occasionally it is used of man (cf. Lk 23:25; Jn 1:13.]
Feel free to search the usages of each, in Scripture.