Facet links created perfectly except with a colon rather than

I think the problem here is that Entity Reference Facet Link is not compatible with Facets 3.0.x and the new recommended way to do Facets filters with Views at all:

New in Facets 3: For new sites it is recommended to use the "Facets Exposed Filters" submodule for Facets on Views. This module uses native Views filters instead, which has many advantages.
Click here for documentation on how to use this new workflow.

What it was picking up is the old-style facets i still had which are not needed nor used with Views now.

Opened a support/documentation issue in Facets looking for guidance, 💬 Upgrading Facets ecosystem module to Facets 3 Active

That's not good. Thank you for reporting it.

I don't have any projects using Facets 3 yet. I'll have to set something up from scratch.

I have a bad feeling that this will force a complete rewrite of ERFL.

Uh, yeah. This is a problem. Don't expect a quick fix. It's going to require something completely new to be developed. TBH, there's probably an argument that it should be done as a separate module.

I'm going to update the project page to note that it's incompatible.

You might see if https://www.drupal.org/project/entity_ref_filtering_link → works for you.