Pull Up All Laminate Boards Or Cut One Board in Doorway and Start From There
You don't have to do anything.  Most laminate installs have a soft spot somewhere.  I also don't think the soft spot will change.  You either accept it or fix it.  I am not saying you had a "great" install but it is what it is.

If you are going to pull it up - you need to do a big enough area where it can be properly leveled.  For issues like this we use a simple leveling liquid or we use roofing shingles (really small transitions).  I cannot see you being happy doing this from below.  Injecting the floor is never a good idea as the weight of the flooring can alter the liquid disbursement.   If you are going to take stuff out you have to label each piece well and take everything out very carefully.  The tongue and groove on these aren't made for many installs.