Lead acid and AGM are the same thing and I agree caution about dod ranges. At 80% dod you'd be doing well to get 300 cycles out of them.
Lead carbons, whilst still the same basic chemistry, are a whole world different as I expect you have already noticed.
You quoted 6k cycles earlier
Are you able to say at what dod this number of cycles refers?
We'd be looking at just under 16½ years life expectancy.
That's a huge number of cycles and well worth the investment if the price is right.
Remaining capacity and hence life expectancy of batteries is based on the number of cycles at a particular dod. Once the capacity has dropped to 80%, that is when the battery is considered to be at the end of it's useful life.
Have you looked at flow batteries? they can go on forever at 100% dod. They will occasionally, after many years, require membrane replacement and maybe electrolyte top up. The only downside for us mere mortals is the mortgage needed for the original purchase.