I assume that you are using 'submission_url' because you want Survey123 to send the submissions to a previously-authored layer in AGOL. Is this correct?
I would let Connect publish the layer by itself (i.e. removing the 'form_id', and 'submission_url' values). If you need to populate the S123-created feature layer with features from your pre-existing feature layer, you can do that afterwards with Pro, or the Python API.
Does it make sense? (I think we are missing some information: why do you need to work with an existing feature layer instead of letting Survey123 to create it from Connect?)