It sounds like you are trying to open yourself up for fisting by using butt plugs, going by the " low bulb to neck ratio" comment. This is not the best way to do this as your anus will contract back around the small neck, meaning you will have to keep pushing it in and pulling it out to stretch the anus it's self; it's the anus that has to be able to open far enough for fisting, not so much the rectum.
You would be better with something to open up the anal sphincter it's self. maybe this would do the job as it starts relativley smaller than the base, so that you can use the one tou to get all the way there, just a bit at a time . https://www.lovehoney.co.uk/product.cfm?p=17540
it's PVC so you should be OK to use an oil based lube, as the last longer, I would also get a lube applicator to get the lube up inside where its needed as much as on the outside.
Also when i say low bulb to neck ratio what i mean is that my anus wont contract far. I mean a plug that doesnt have a big difference just enough to stop it from falling out.