Was Jesus adopted as God's Son at his baptism or resurrection?

No. Read in context, the verses that can sound that way describe God declaring, confirming and enthroning one who was already his Son, not adopting a man. At the baptism the voice says "You are My beloved Son" (Mark 1:11), without Psalm 2's "today I have become Your Father." Luke, who records Peter's "God has made this Jesus, whom you crucified, both Lord and Christ" (Acts 2:36), applies Psalm 2 to Jesus before Easter (Acts 4:25–27), so Easter is not where Jesus becomes Messiah, and he has the angel call the newborn Jesus "Christ the Lord" (Luke 2:11). And Paul's "declared with power to be the Son of God by His resurrection" is said of one who is already "His Son, who was a descendant of David according to the flesh" (Romans 1:3–4). Bart Ehrman argues that some of these lines preserve older formulas that did mean adoption; that is a debated reconstruction of a layer behind the texts, not what the texts as we have them say.

What these verses describe is real. Exalted to God's right hand, the risen Jesus pours out the Spirit (Acts 2:33) and receives "all authority in heaven and on earth" (Matthew 28:18). The exaltation brings him, as the risen man, into the open exercise of a lordship that was always his. It gives him no new nature, no new sonship and no higher divine rank; read with the rest of the New Testament (John 1:1; Philippians 2:6; Galatians 4:4), his deity and sonship are eternal. The deep dive takes up Ehrman's case in full, and why the New Testament as a whole tells against it.