...This covers a little over 1/2 the filter in a 5 gallon bucket. I will soak for several hours, and then turn filter upside down. Is that OK?...I will save cleaning solution and reclean spare filter every 30 days or so, and save solution for up to 1/2 year for 4-5 more cleanings.
My concern with soaking only half the filter at a time is that any material that was loosened may dry back on the filter while the other half soaks. I would either find a smaller container that would allow your solution to cover the whole filter, mix up more so that it would be covered in your 5 gal bucket, or at least rinse off the soaked side when flipping the filter. IMO, since you plan to re-use the solution, using more won't cost that much -- although 6 months sounds like a long time to keep the solution.
FYI, if you are concerned about the cost of the filter cleaner, I use 1/2 cup of TSP in enough water to cover my filters in a 5 gal bucket. It seems to do a good job. I have heard that others have had success using dishwasher detergent.