Hello and welcome to the forum
It’s quite tricky to use a sub-account as a catch all account. It should be possible to use filters in the main account to move incoming mail to known aliases to the Inbox and then use a filter at the end of the filter list to redirect anything else addressed to your domain to the sub-account.
You would need the filters to be listed in a particular order and for this you use the order values on each filter. The first filters would need to look for known addresses on your domain for that main account and move the messages to the Inbox. Then a filter looking for @domainyouown.com in the headers to redirect everything else to the sub-account.
It’s because of this last filter you need the former ones otherwise everything in the main account would be forwarded to the sub-account.
Other sub-accounts would not be affected as mail addressed to them would go directly to those accounts.
If you want to give this a try and need some help I would suggest you open a support ticket so we can look at your filters once you’ve set them up.