Codeine has a roof limit of about 400-600mg, You are already approaching the roof limit and if you do feel so little its likely you either do not or cannot convert the drug nearly as well if not at all anymore as you use to, this is possible and about 15% of the population to begin with have issues with not being able to properly convert codeine and some even over convert it hence making OD more possible, In your case you most likely wont OD in the respiratory depression sense but there is a great chance your shooting your kidneys through hell right now. I'm never here to judge or anything of the sort but all I can do is suggest you stop, and probably best you don't cold turkey such dosages.
This is also unlikely and I recognise that so all that I could then further suggest for your health is you find something that is not going to force you to put such extreme amounts through your body, but this is the very start of that vicious cycle. I wish you luck and hope the best for your future. I do not condone jumping to a more powerful drug but if you must pick the worse of 2 evils that will save your body in the short term and maybe you wont have a toxicity complication say like the late Juice wrld. But changing to a stronger opiate is a harder addiction to kick and of course has its own down sides. Codeine may not again OD your respiratory system but a stronger opiate just might.
