Yes, destination goal looks the most appropriate for your scenario. Because you have many different potential funnels, I would stick with a single goal for all these potential paths - just define the thank you page as your goal condition.
In your analysis, I would look at specific reports to help you determine the success of different pages.
One report you could check out is Conversions > Goals > Reverse Goal Path. This will show you the most popular routes that visitors took to complete your goal.
Another report you could look at is Behaviour > Site Content > Landing Pages. In this report, select the right goal in the Conversions section of the table.
To get Organic data only, I would suggest applying a Segment to show Organic traffic only.
Unfortunately, it will be difficult to see keyword data in your reports because Google Analytics is very limited in collecting the data in the first place. Knowing the landing pages though will give you a good idea of user intent.
If you had fewer potential goal paths, I would suggest a different approach and have a different goal for each funnel. Best approach would be to use GTM and rewrite path name for each scenario. I've written a step-by-step tutorial on how to do it here: