Bob,
With 155,000 messages and well over 3000 members, a global search
would simply generate far too large a load of garbage that no one
could possilby use in our group
That seems like an odd assumption.
Yes, it would if one searched for a word or phrase that is too commonly used in your group, but that's what search refinement is all about.
After all, web search engines like Google and Bing index untold billions of pages, yet people routinely get useful search results from them. I doubt Groups.io's search relevance ranking is anywhere near to being in the same sophistication ballpark as the major engines, but then the universe of content (messages + photos + files + ...} in a single group isn't anywhere near to being in the same size ballpark as the entire web.
Shal
Shal Farley