RE: I would like to get a view of all marketing form submissions in one leads view
Hi Patrick,
Source form field of lead will be only populated when the lead is created from form submission, whereas your leads may not be generated from the submission.
In Dynamics 365 Marketing, there are two types of data: Profile data and Interaction data.

Form submissions is interaction data, they will be saved to marketing-insights service database instead of organizational database, therefore, they couldn't be queried by Advanced Find.
To create a view to see form submissions, there are three options:
1. Connect to marketing-insights service database with Power BI:
https://docs.microsoft.com/en-us/dynamics365/marketing/custom-analytics
2. Set the "Store form submission" field to Yes in Marketing form > Summary > Submission behavior.
By enabling the option, it will create form submission records in organizational database at same time(keep all incoming form submissions), therefore, we can query records of "Marketing form submission" entity with Advanced Find to see form submission in view.
3. Make the marketing form only collect form submissions without updating contacts or leads.
https://docs.microsoft.com/en-us/dynamics365/marketing/marketing-forms#do-not-createupdate-contacts-or-leads
Regards,
Clofly