How to Add Indexed Property to a Site So I can build a result source to search all sites for content with this property

I want to build a sharepoint online search result source that includes only some site collections and subsites. I have over 5000 Site collections, so I can't use named URLs/site ids in the result source - not maintainable. Instead I hope to add an indexed property to a site's property bag after site is provisioned then map a managed property to it, and use it in a resultsource to filter search results to match only content found in sites that have that property value. The Phil Harding's article describes the approach: https://platinumdogs.me/2015/02/06/set-a-propertybag-property-as-indexed-queryable-via-search-using-csom-powershell/ and Mike Morawski adds some code for the indexed property encoding - http://www.migee.com/2015/09/14/allowing-property-bag-values-to-be-searched-via-sharepoint-search/ I used bits of both to implement this approach.

Approach:

1. Add Key = 'SiteType', Value = 'MySiteType' to Web All Properties
2. Add Indexed Property 'SiteType' with 'MySiteType' Value to web IndexedProperties (vti_indexedpropertykeys). Key encoded to base64
3. Add Managed Property 'propSiteType' mapped to 'SiteType' Crawled property in Search Schema manually.

I've done 1 and 2 via Powershell+CSOM, and verify site property added and is crawled. Managed Property is there, but It is not available in ResultSource builder dialog, and any searches such as {searchterms} propSiteType:MySiteType or (contentclass:STS_Web OR contentclass:STS_Site) propSiteType:MySiteType do not return results.

Ideas or alternative approaches? Thanks in advance

My only thought is an indexing delay. I have seen O365 take days to index new content, and that's even when manually requesting a crawl. If that was the issue, it's probably resolved by now. Are you seeing relevant search results?

https://www.sharepointnutsandbolts.com/2013/10/waiting-for-search-crawl-in-office-365.html