There are lots of ways to produce links based on routes in Drupal, but i cannot easily see what i expect should be a simple pattern to determine the route name for the edit page of a form mode after looking through the code under Routing for EnhanceEntityRouteSubscriber, FormModesSubscriber and under Plugin where the routes would be used to create links like FormModeManagerContextualLinks, and at #2700537: "No link template 'xxx' found for the 'xxx' entity type" →
… i thought i'd ask and suggest we document this.
Active
3.0
Documentation
"entity.$entity_type_id.edit_form.{$this->formModeManager->getFormModeMachineName($form_mode['id'])}"
looks like it, but we'd want to document the easiest way to get the form mode machine name and how to pass in the individual entity (ID) as a parameter when using the route to get a link to the entity's edit form mode.