How to retrieve extended properties for calendar event (by name)?

I have an Exchange calendar populated though Exchanges Web Services (EWS) UpdateItem SOAP calls containing:
(Example here for an integer TTID and a TTSyncID string field)

 <mes:UpdateItem ConflictResolution="AlwaysOverwrite" SendMeetingInvitationsOrCancellations="SendToNone">
    <mes:ItemChanges>
       <typ:ItemChange>
          <typ:ItemId Id="%s" ChangeKey="%s" />
          <typ:Updates>
             <typ:SetItemField>
                <typ:ExtendedFieldURI DistinguishedPropertySetId="PublicStrings" PropertyName="TTID" PropertyType="Integer"/>
                <typ:CalendarItem>
                   <typ:ExtendedProperty>
                      <typ:ExtendedFieldURI DistinguishedPropertySetId="PublicStrings" PropertyName="TTID" PropertyType="Integer"/>
                      <typ:Value>10</typ:Value>
                   </typ:ExtendedProperty>
                </typ:CalendarItem>
             </typ:SetItemField>
            <typ:SetItemField>
               <typ:ExtendedFieldURI DistinguishedPropertySetId="PublicStrings" PropertyName="TTSyncID" PropertyType="String"/>
               <typ:CalendarItem>
                  <typ:ExtendedProperty>
                     <typ:ExtendedFieldURI DistinguishedPropertySetId="PublicStrings" PropertyName="%s" PropertyType="String"/>
                     <typ:Value>SomeStringData</typ:Value>
                  </typ:ExtendedProperty>
               </typ:CalendarItem>
            </typ:SetItemField>
          </typ:Updates>
       </typ:ItemChange>
    </mes:ItemChanges>
 </mes:UpdateItem>

In EWS they came back in the GetItem SOAP requests as

  <ExtendedProperty>
    <ExtendedFieldURI DistinguishedPropertySetId="PublicStrings" PropertyName="TTID" PropertyType="Integer"/>
    <Value>10</Value>
  </ExtendedProperty>
  <ExtendedProperty>
    <ExtendedFieldURI DistinguishedPropertySetId="PublicStrings" PropertyName="TTSyncID" PropertyType="String"/>
    <Value>SomeStringData</Value>
  </ExtendedProperty>

I'm now trying to retrieve these with the other event properties through Postman calls to the MS Graph API.

From what I understand I have to use Outlook extended properties and not Open extensions to keep this backward compatible (correct me if I'm wrong).

But I can't get this to work, because the singleValueExtendedProperties expansion seems to insist on MAPIPropertyTpes and GUIDs. If I call

https://graph.microsoft.com/v1.0/users/{{UserID}}/calendar/events/{{TTSyncedEventID}}?$expand=singleValueExtendedProperties($filter=id eq '{{extensionID}}')

with {{extensionID}} = TTSyncID or TTID, I get

{
  "error": {
    "code": "ErrorInvalidProperty",
    "message": "PropertyId values may only be in one of the following formats: 'MapiPropertyType namespaceGuid Name propertyName', 'MapiPropertyType namespaceGuid Id propertyId' or 'MapiPropertyType propertyTag'."
  }
}

Can I somehow retrieve these namespaceGuid and use those (as suggested here?
Is there an easier way referencing the properties with the human-readable TTSyncID or TTID?

Based on this, namespace GUID for public strings property sets is {00020329-0000-0000-C000-000000000046}.

If you use the following form of URL

https://graph.microsoft.com/v1.0/users/{{UserID}}/calendar/events/{{TTSyncedEventID}}?$expand=singleValueExtendedProperties($filter=id eq 'MapiPropertyType namespaceGuid Name propertyName')

You should be able to get the value of TTID property

https://graph.microsoft.com/v1.0/users/{{UserID}}/calendar/events/{{TTSyncedEventID}}?$expand=singleValueExtendedProperties($filter=id eq 'Integer {00020329-0000-0000-C000-000000000046} Name TTID')

and get the value of TTSyncID property

https://graph.microsoft.com/v1.0/users/{{UserID}}/calendar/events/{{TTSyncedEventID}}?$expand=singleValueExtendedProperties($filter=id eq 'String {00020329-0000-0000-C000-000000000046} Name TTSyncID')