I have not done The Great Loop yet, but I plan to when I retire, so your question is also important to me.
The most important thing is proper surface preparation so the paint will have good adhesion. This really matters if your boat will be in the water for up to a year during the Loop. If most of the old paint is worn off, you may want to clean all the old paint off down to the hull. One option is Soda Blasting (NOT sand blasting). This does a really good job of cleaning, but I think it must be done by a professional that specializes in boats. I had this done on a previous boat and it was $40 per foot, or $1,000 for my 25' boat. Sanding may be another option. If you do it yourself, be sure to wear a proper face mask: you don't want to inhale the dust and copper particles from bottom paint. I tried paint removers for small areas, but it was messy and did not work very well.
Then, it is good to apply a barrier coat/primer that will provide good adhesion for the bottom paint. The first coat of bottom paint must go on within a certain time window after the primer is applied. I also had this done on my previous boat with Interlux products and it was also $1000. Prices are probably higher now due to inflation.
For my new R-27, the dealer is putting on Pettit Protect for the barrier coat/primer followed by Pettit ViViD bottom paint. I have not used this paint before, but have been researching it. It has dual biocides to deal with slime and barnacles and is a "hybrid" paint that is supposed to have the properties of a long wearing hard paint and an ablative paint. The dealer says this works well for New England boating where we boat in salt water and brackish to fresh water on the coastal rivers. These paint properties might also be good for the loop which is about half fresh water and half salt water. Other paint vendors have similar products. I am not trying to just push Pettit or Interlux.
Probably the best suggestion is to join the AGLCA (America's Great Loop Cruiser's Association). They have a forum where you can ask actual loopers what worked best for them.
Good Luck,
and let us know what you find out.