Hi Amelie,
The trigger will just split your members in start segment into different path in customer journey,

your recipents will just go through to not interested path and what we could do is adding Actions tile for them,
and these actions will only perform on them. (While interested contacts go throught another path)
To collect recipents who not submit form for a period of time, we could instead create an activity for them,
these activities records regarding will be set to contacts in customer journey automatically.

then we can find filter these contacts in Advanced Find.
Regards,
Clofly