I'm not 100% sure if creating model annotations in assemblies through the API is fully supported yet, like it is for parts. The 2021 Object Model only had the ModelAnnotations object shown under the PartComponentDefinition, and not under the AssemblyComponentDefinition, but I do see now that there is an online help page for it under the assembly definition.
It's definitely a lot more complicated than just adding a general note to a drawing sheet, but it can be done with some practice. I haven't done a ton of it myself by code yet, but I threw together the basics in an iLogic rule for you, just in case you may still be interested in going that 3D annotations route down the road. I only went into creating either a leader note or a general note, but there are several others.
Dim oPDoc As PartDocument = ThisDoc.Document
Dim oPDef As PartComponentDefinition = oPDoc.ComponentDefinition
Dim oMAs As ModelAnnotations = oPDef.ModelAnnotations
Dim oFText As String 'FormattedText you want the note to show
'for a leader note
'define a plane for the leader note to reside on in 3D space
'oPlane = '(Planar Face or WorkPlane)
'oXAxisDirection = '(linear edge, work axis, or 2D sketch line) (not required)
Dim oAPlane As AnnotationPlaneDefinition = oMAs.CreateAnnotationPlaneDefinitionUsingPlane(oPlane, oXAxisDirection)
Dim oLeaderPoints As ObjectCollection = ThisApplication.TransientObjects.CreateObjectCollection
'oLeaderPoints.Add() 'add points to the collection for the leader
Dim oMLNDef As ModelLeaderNoteDefinition = oMAs.ModelLeaderNotes.CreateDefinition(oLeaderPoints, oFText, oAPlane)
Dim oMLN As ModelLeaderNote = oMAs.ModelLeaderNotes.Add(oMLNDef)
'for a general note
Dim oMGNDef As ModelGeneralNoteDefinition = oMAs.ModelGeneralNotes.CreateDefinition(oFText, True, ScreenQuadrantEnum.kLowerLeftQuadrant)
Dim oMGN As ModelGeneralNote = oMAs.ModelGeneralNotes.Add(oMGNDef)
Wesley Crihfield

(Not an Autodesk Employee)