The OP asks, “Is the ‘will’ of God here a statement of who will go to heaven and who to hell?” Given the gist and tenor of Paul’s arguments in Acts 20, the answer is no. The OP’s question implies that God wills for some to go to heaven and others to hell. If the fate of each soul were decided beforehand, there would be no reason for Paul to declare himself innocent of the blood of anyone else. It would also be pointless of him to warn the disciples against the threats to themselves and to the flock that will come. It is precisely because the stakes are high and the outcome uncertain that Paul warns them to remain alert.
Acts 20:26-31 NIV
26 Therefore, I declare to you today that I am innocent
of the blood of any of you. 27 For I have not hesitated
to proclaim to you the whole will of God. 28 Pay
careful attention to yourselves and to all the flock, in which the
Holy Spirit has made you overseers, to care for the church of God,
which he obtained with his own blood. 29 I know that
after my departure fierce wolves will come in among you, not sparing
the flock; 30 and from among your own selves will arise
men speaking twisted things, to draw away the disciples after them.
31 Therefore be alert, remembering that for three years
I did not cease night or day to admonish every one with tears.
The OP points out that in Acts 4:28, boulé is something that is decided beforehand. The noun boulé (βουλή Strong’s 1012) means counsel or purpose. Per Thayer’s Greek Lexicon, boulé is especially used of God’s purpose with respect to Christ and the salvation of men.
Thayer's Greek Lexicon: boulé
especially of the purpose of God respecting the salvation of men
through Christ: Luke 7:30; Acts 2:23; Acts 4:28; [Hebrews 6:17]; πᾶσαν
τὴν βουλὴν τοῦ θεοῦ all the contents of the divine plan, Acts 20:27; ἡ
βουλὴ τοῦ θελήματος αὐτοῦ the counsel of his will, Ephesians 1:11.
Ephesians 1:8-12 [brackets added]
With all wisdom and understanding, 9 he made known to
us the mystery of his will according to his good pleasure, which he
purposed in Christ, 10 to be put into effect when the
times reach their fulfillment—to bring unity to all things in heaven
and on earth under Christ. 11 In him we were also
chosen, having been predestined according to the plan of him who works
out everything in conformity with the purpose [boulé] of his will,
12 in order that we, who were the first to put our hope
in Christ, might be for the praise of his glory.
In Acts 4:28, we see that God can arrange all things to realize His purpose [boulé]. The noun boulé originates from the verb boulomai (βούλομαι Strong’s 1014 meaning to will). The verses in which these words are found form a consistent picture of God’s will that no one should perish and His purpose that all may live through Christ.
2 Peter 3:9
The Lord is not slow in keeping his promise, as some understand
slowness. Instead he is patient with you, not wanting [boulomai]
anyone to perish, but everyone to come to repentance.
Though God can make all things conform to His purpose, to my knowledge, He does not force His will/bouléma (βούλημα Strong’s 1013) when it comes to the fate of each soul. The way boulé is used in Luke 7:30 is evidence that men can and do reject God’s counsel for themselves.
Luke 7:30
But the Pharisees and the experts in the law rejected God’s purpose
[boulé] for themselves, because they had not been baptized by John.
Returning to the OP's question, the word boulé as used in Acts 20:27 is understood to be a reference to God’s singular and unchanging purpose for us in Christ. If God has a purpose for some to be saved and a separate purpose that others should perish, the details of the latter are nowhere to be found. Beyond lexical considerations, for God to have such opposing purposes would, I believe, go against His very nature (cf Ps 145:9, Ez 33:11, Jam 1:17).
Keil and Delitzsch commentary on Psalm 145:9
The real will of God tends towards favour, which gladly giving stoops
to give (חנּוּן), and towards compassion, which interests itself on
behalf of the sinner for his help and comfort (רחוּם). Wrath is only
the background of His nature, which He reluctantly and only after long
waiting (ארך אפּים) lets loose against those who spurn His great
mercy. For His goodness embraces, as Psalm 145:9 says, all; His tender
mercies are over all His works, they hover over and encompass all His
creatures. Therefore, too, all His works praise Him: they are all
together loud-speaking witnesses of that sympathetic all-embracing
love of His, which excludes no one who does not exclude himself