Case Insensitive Search With Regex - regex

Question

I'm trying to implement a case-insensitive search with regex.
Example: /^sanford/i (searching for anything starting with "sanford" disregarding case sensivity.
For case insensitive queries, creating indeces with a custom collation is recommended by the documentation. This works fine as long as no regex is involved.
The problem: searching with a regex (in this case "starts with"), the case-insensitive search does NOT take the index into account.
This is stated in the documentation multiple times and is also reproducable with an explain command.
To sum it up: It works, but without effectively using the index. I'd be glad to get any hints, I can't get rid of the feeling that I'm missing something fundamentally important here.
Inserting the string with toLowerCase and then searching only with lower cased strings is not an option.
I can't use a text index because there can only be one per collection.
Example from the documentation see here, the green info box on the bottom.
#D.SM: The index is used, but it scans all documents.
https://docs.atlas.mongodb.com/schema-suggestions/case-insensitive-regex/
Example document:
{
"name": [{
"family": "Test",
"given": "Name",
}],
}
Index with collation:
{ "name" : "name_family", "key" : { "name.family" : 1 }, "host" : "noneofyourbusiness.com", "accesses" : { "ops" : NumberLong(114), "since" : ISODate("2020-07-30T20:25:59.079Z") }, "shard" : "shA", "spec" : { "v" : 2, "key" : { "name.family" : 1 }, "name" : "name_family", "ns" : "noneofyourbusiness.somethingwithaname", "collation" : { "locale" : "de", "caseLevel" : false, "caseFirst" : "off", "strength" : 1, "numericOrdering" : false, "alternate" : "non-ignorable", "maxVariable" : "punct", "normalization" : false, "backwards" : false, "version" : "57.1" } } }
}