I don’t think that there is an easy solution for this. The simplest straightforward solution is a microflow that does a retrieve per entity per attribute that you want to have included in the search. But that will be dead slow as soon as you have some serious amount of data.
Second: prepare and maintain an index entity having attributes: textfield, objectId, entityname, attributename. The search will be straightforward: a retrieve on the index entity where textfield contains the searchtext. Downside it that maintaining the index requires you, for any entity-attribute that will be included in the search, to write any change to the index. But it can be done.
Third option is probably overweight for the requirement, but I let you be the judge of that: install some software that is excellent in searching large datavolumes. For instance the ELK-stack.