Hi Burly,
-> For your basic situation:
form A and form B, Customer Journey A and Customer Journey B,
you could take following process as reference:
1. Select Submitted a form during journey template to build a segment, this would be suitable for what you've described.
Or build it from scratch by clicking Behavior type segment.

2. This segment will find contacts who have submitted "My subscription center" between 1/6 yo 1/31 during "0108-02" customer journey.

adminer mao has been found, because I let him changed his firstname name via the form on 1/8.

3. In a new customer journey: 0109-01, we added the previous segment to Suppression segment field,
then "adminer mao" will quit new journey even if he had been included in segment that was added in the new journey.

-> For your ultimate goal
Due to each customer journey could add only one suppression segment,
so my thought is that create a combined segment.
For example, combined contacts who have participated cj1 and contacts who have participated cj2,
combine them in a new segment, then add it to supression segment in cj3.

Reference:
How to design behavioral segments:
https://docs.microsoft.com/en-us/dynamics365/marketing/segments-interaction
How to use combined segments:
https://docs.microsoft.com/en-us/dynamics365/marketing/segments-compound
Regards,
Clofly