Was having another thought…but to the above, the reason why it’s not working is that we would need to convert the unix timestamp into a hex before passing to $toObjectID, i.e.

Current time as Unix: 1711789762 769
Create an objectID now: 66 07 d6 97 8b e6 b1 6f 5b 23 8a 85

Using the above site, we can see that this is broken down as:

TT TT TT TT MM MM MM PP PP II II II
66 07 d6 97 8b e6 b1 6f 5b 23 8a 85

Where:
TT : Unix timestamp as Hex representation
MM : Machine ID as Hex representation
PP : Process ID as Hex representation
II : Incrementing Number as Hex representation

So we’d need to convert 1711789762 to a hex representation, which we could do but at this point I think it’s getting way too complex, if you’re calculating a field to compare for every record the performance will be horrible.

You can normally compare an object ID against a date, convert the ObjectID to a date and you can compare.

Reformatting your query:

{
   "$lookup":{
      "from":"leads",
      "let":{
         "minID":{
            "$toObjectId":"$dateEmailed"
         },
         "post":{
            "$toObjectId":"$post"
         },
         "member":"$member"
      },
      "as":"leads",
      "pipeline":[
         {
            "$match":{
               "$and":[
                  {
                     "$expr":{
                        "$gt":[
                           "$_id",
                           "$$minID"
                        ]
                     }
                  },
                  {
                     "$expr":{
                        "$eq":[
                           "$post",
                           "$$post"
                        ]
                     }
                  },
                  {
                     "$expr":{
                        "$eq":[
                           "$member",
                           "$$member"
                        ]
                     }
                  }
               ]
            }
         }
      ]
   }
}

Have you tried converting your ObjectID to a date as opposed to creating an ObjectID from a date?

I may have made errors above in regards to the casting and number of digits needed but I think the gist is right. I think the generation of an ObjectID is probably trying to drive a nail into a plank of woof by using a sledge-hammer attached to a see-saw.
(I tried to use AI to generate that image, but it seemed to REALLY confuse the AI!)