There are a lot of options here. First, if you are using a version of Word that supports content controls then there is really no need to be populating bookmarks. You can populate content controls instead.
As for the rich text, you can either define the rich text content as building blocks and insert it in a target rich text CC, or simply have it a boiler plate content in the template and delete it if not needed:
here is some sample code:
Sub Macro1()
Dim bCheck As Boolean 'simulates your checkbox
Dim oTmp As Template
Dim oCC As ContentControl
Dim oRng As Range
bCheck = False
Set oCC = ActiveDocument.SelectContentControlsByTitle("RTTarget").Item(1)
If bCheck Then
Set oTmp = Templates("D:\My Documents\Word\Templates\Normal.dotm") 'Or whatever template defines your buildingblock. I used my normal.dotm and a BB titled RT1
'Use a target RichText contentcontrol titled e.g., RTTarget
oTmp.BuildingBlockEntries("RT1").Insert Where:=oCC.Range, RichText:=True
Else
Set oRng = oCC.Range
oCC.Delete True
oRng.Paragraphs(1).Range.Delete
End If
End Sub
Sub Macro2()
Dim bCheck As Boolean 'simulates your checkbox
Dim oTmp As Template
Dim oCC As ContentControl
Dim oRng As Range
bCheck = False
If Not bCheck Then
Set oCC = ActiveDocument.SelectContentControlsByTitle("RTTarget").Item(1)
Set oRng = oCC.Range
oCC.Delete True
oRng.Paragraphs(1).Range.Delete
End If
End Sub