We do a similar thing since our area of interest is smallish but spread out over a very large area. Collector is not very good with this and the main reason we use a 3rd party app.
That said I think your option is to do a side loaded basemap (search for posts on how to do this - hopefully you have android since it is so much easier). Build the basemap using the Manage Tile Cache tool using a AOI then package it up using Export Tile Package. Do not count on the Export tool for AOI as it has 4 bugs in it. My bugs are in this post along with a good post on how to - Create Cache and TPK without ArcGIS for Server - Workflows for Desktop users
The issue here may be that Collector still will not like the huge area.
I am not sure 1:1,800 is going to happen. We go down to L17 (1:4,500) for out internal network caches and one of our areas that is 50,000 acres is about 1gb. Again with the bugs if it is really spread out it may be more. Then each level is 4x or so larger so 16gb?. We do L16 on the tablets because after testing it does not really look much better using NAIP. In the end the extra space needed was not worth it. I would build a few small area tests and see. I am also not sure if Collector can "overzoom" like our app and ArcMap can or if it goes to blank like AGO does (which is a bummer). By overzoom I mean show the image but start to pixelate vs just going blank.
We do a topo basemap that covers our entire area (the whole Pacific NW) down to L14 for use between areas and as a nav option (in our app you can turn off the imagery and see under to the topo). The topo is 5gb on its own. Our total mobile package is 29 gb so we use sd cards - which collector does still not allow.
Hope that helps.