blog_20210430_auth_awssdkgo_assume.go

ctx := context.Background()
assumecnf, _ := config.LoadDefaultConfig(
	ctx, config.WithRegion("{aws-region}"),
	config.WithCredentialsProvider(aws.NewCredentialsCache(
		credentials.NewStaticCredentialsProvider(
			"{aws-assume-role-key}",
			"{aws-assume-role-secret}", "",
		)),
	),
)

stsclient := sts.NewFromConfig(assumecnf)
cnf, _ := config.LoadDefaultConfig(
	ctx, config.WithRegion("{aws-region}"),
	config.WithCredentialsProvider(aws.NewCredentialsCache(
		stscredsv2.NewAssumeRoleProvider(
			stsclient,
			"{rolearn-to-assume}",
		)),
	),
)

client := s3.NewFromConfig(cnf)
res, _ := client.GetObject(ctx, &s3.GetObjectInput{
	Bucket: awsv2.String("{some-bucket}"),
	Key:    awsv2.String("{some-key}"),
})

...

Just want to say "thank you!!!" - this was a big help (and somehow not something readily available in the AWS documentation).