a] Do you want custom fields for both Jobs and Experts as well?
Both would be good, JOBS is more important
b] Do you want them only for the back-end and for Administration purposes only, or in the front-end as well?
Front end is vital
c] What would some types of those fields might be? I.e. only text fields, or more complex ones as well?
Complex, ie checkboxes would be good, TEXT FIELDS is acceptable
d] Would it be ok to simply "add extra" fields or you want by 100% to edit the existing ones as well?
ADD EXTRA FIELDS is ok
Also, as an extra note, both Jobs and Experts are Post Types, so any "built-in" WordPress Custom Field or via a plugin like Advanced Custom Fields etc could be added by you as well, but not on the same "metabox" as the existing ones and it will be a bit tricky to show them in the front-end.
Some more information on this aspect would be helpful
max_execution_time set to 300
All plugins except WPMU DEV Dashboard, Appointments, and Google Maps Pro plugin disabled
Basic Twenty Nineteen theme installed and the problem still exists.
I also deleted and reinstalled Google Maps Pro but with no change.
To be clear, with only 3 plugins enabled, WPMU DEV Dashboard, Appointments, and Google Maps Pro, the site breaks . When Google Maps Pro is disabled the site works.