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You need more conversion data to find qualified clicks. Think about bidding to a more upstream goal, like simply getting to your form fill or a page depth of 2+, in addition to your lead form submit.
If you use tROAS, you can weight these actions according to their conversion value (100 form fill page visits converts to 1 lead form submit, so they are worth 1/100 of your lead form, etc).
If you are more comfortable with tCPA, use them all (and set your conversion action to count all, not one per session). That will help to weight the clicks that do convert to leads.
This will take some time to build up, make sure your conversion pixel has a long conversion window opened up as well
