Good morning Colby, Your base thickness sounds appropriate for your climate ( this is what we use and we are also located in N. CA.) I'd go closer to the 3" rather than the 2" for the 3/4" crushed gravel. If you have access to 3/4" class II base rock that would be preferable to straight gravel. Class II base has some fines mixed into the gravel that aids in binding the base together when it is compacted. Additional recommendations , : compact the soil below your sub base. Especially if it is highly organic. Loamy soil to clay like soil is made up of decomposing matter that continues to decompose as it ages. This is one example of why dry lay patios eventually become uneven and sink , it is one of the reasons why the topsoil layer is commonly removed when laying a patio. I support the filter fabric method between the compacted gravel base and the sand layer, especially if you use straight gravel which even when it is compacted still has minimal air gaps for the sand to filtrate downwards. 1x6 plastic border will warp out of shape very quickly once a downward and sideward load is pushed against it. You will need to either augment the 1x6 with additional shear or additional equal loading pressure or use a more appropriate size thickness to support the pressure. Some methods may include digging a perimeter concrete footing - sort of like a curb that is located below and under the edge of the flagstone. Or beefing up the edging to a 2x material or countering the side pressing load by edging the edging with rock or cobbles or some other stable + grounded methods.

I just built a quartzite patio in Reno, NV. I used the new product, polymeric sand and it worked great! I got the sand in 60 lb bags for $20 a bag from Home Depot. My patio is 9X27.5 and I used 5 bags. It looks like regular sand and sweeps into the joints. You then mist it 3-4 times waiting about 10 minutes between each misting. The water activates a weak polymer glue and locks the sand granules in place. We don't have much problems with frost heaves here, but if a stone moves you can pull up the sand in chunks and step on it or use a shovel to get it back to a granular stage, sweep it back in place and re-mist. Although the polymeric sand sets up pretty hard I don't think it would stand up to a leg from a patio chair, etc., so we plan on getting sled style chairs for the patio. It does do well with walking on it and the sand stays put! So far we are very happy with it. I'll let you know how it goes thru the winter. We get very little rain here but we do get snow.

You should be able to get samples for whatever material you are considering. Place them in the sun in various colors so you can feel the difference. Depending on your location, sun exposure, whether any shade is available...all these factors make a difference. (Certain types of flagstone might have other issues like its reaction with chlorine water that may factor in). Good luck - there are tones of options!

Following, so bumping. BTW, not sure I fully understand needing a step down - people have walk out, accessible patio's and decks ....how are their's different? I'm looking to add a wheelchair roll out raised patio and am hoping to learn more...I know I'll need to protect the entry with a deep overhang....hoping you get some knowledgable responders.