Hi again @kevinadi thank you so much for all the very well-explained tips so far!
Regarding the current status I have the below as per your questions:

I used the below command

 test -e /var/lib/mongodb/journal/WiredTigerLog.0000000015 && echo file exists || echo file not found
file not found

so it seems missing.

Well, I am the only person handling this database, hence working on the mongod.conf file.
I had once changed the mongod.conf setting by uncommenting the below
# engine: wiredTiger
and then commenting it again but I have not used something more as extra parameters to WiredTiger, is there any way to go on with that?