This is what I did, which works for my purposes because I only have one contribution form.
Added a temporary redirect in htaccess for the Collecting module’s success view:
Redirect 302 /collecting/1/success https://example.com/s/archive/collecting/1/success
Edited the submission form’s action attribute so that it doesn’t use CleanUrl’s url structure. To edit the action attribute, I edited Collecting/src/Api/Representation/CollectingFormRepresentation.php
In the getForm() function, I replaced this:
$form->setAttribute('action', $url('site/collecting', [
'form-id' => $this->id(),
'action' => 'submit',
$form->setAttribute('action', '/s/archive/collecting/' . $this->id() . '/submit');
This solution works for me, but it is an edit to the module’s core file, so I’ll have to remember to edit this after Collecting module updates.
If anyone has a cleaner solution, I’m interested!