There is no real "System" ...
IF, you know where the 'holes' are; THEN, just work from ANY one of the 'holes'.
Then, when you have done as much as you can with the 'hole' that you have been working on, just pick another 'hole'; and, go from there - etc.
Plus, there is nothing wrong with going over things often.
Even, where the work that is (supposedly) complete.
NEW "Sources (ie. records) are becoming available just about every day, I quite often, find; and, add, NEW "Sources (ie. records) to my Family/Ancestors that are (supposedly) 'Done' (so to speak).
Genealogy/Family History is NEVER complete.
Life goes on ...
Genealogy/Family History is never-ending ...
Just my thoughts.
You might want to try https://puzzilla.org/. It can pinpoint descendant research you can do and enable you to identify a child who may have died young and did not marry, so you know that there are no descendants to try from that person.
I would also suggest descendcy research but I dont use a 3rd party app. Use the fan chart and go back several generations..the outer loop often for me. Bring up that person page and then look at his descendents view 3-4 generations out. Then look for the holes and incomplete things. Is there death dates missing? Should that person have a spouse? Look at the record hints and problem icons and add sources. Find them in all the census record you can. That is where I start and I can stay pretty busy. Finding info in the 1840-1920 records is fairly easy so try to start in that area and learn and then move back.
I do this too, and go back from time to time to check again as there are so many records being indexed and added to potential record matches that sometimes I am able to work forward several generations where a year ago I found nothing. I also fill in non-relative spouse ancestors when I find the information for them.
Thank you! That has been helpful.