Need to plug a variable into a elasticsearch query

I am trying to roll my own search_after pagination query by saving the last sort values to a db and then retrieving them on the next time I run the query.  Thing is if there are no sort values from the db this means this is a new query and I skip the search_after clause in my query. If I do retrieve a set of sort values from the db then I need to plug them into the query. I am having trouble with the EL statement. This is what I have:

{
	"query": {
		"bool": {
			"must": [
				{"term": {"key1": "value1"}}
			],
			"must_not": [
				{"exists": {"key2": "value2"}}
			],
			"filter": [
				{"range": {"@timestamp": {"lte": "now-1w"}}}
			]
		}			
	}
	${search_after:isEmpty():not():ifElse(', \"search_after\": "${search_after}"', '')},
	"sort": [
		{"@timestamp": "asc"},
		{"value3": "asc"}
	]
}

 The ${search_after} variable defined as  '[ts int milliseconds, "a key value string"]'  Including the sq brackets.

The error I am receiving is:
Unknown key for a VALUE_STRING in search_after line 15 col 20 type parsing exception

Does anyone know how to write the variable correctly?