Very long battery life with rechargeable lithium ion batteries
Start with the manufacturers datasheet (if it does not exist, try emailing them, sometimes they just don't have it in an obvious location), this should include things like discharge cruves at different loads, ESR vs temperature, and other data that can help you plan for your exact device (small lithium like CR2320's ESR shoots up like a cliff when the temperature gets too low), be aware these values will be similar but not exact between similar size, capacity and C rating batteries, e.g. https://data.energizer.com/pdfs/lithiumcoin_appman.pdf


Take these graphs and work with them to make a calculator, I have made on for most of my previous projects in excel, decide on a clock speed, then from there enter in the expected power consumption values and work from there, its usually along the lines of (mode1 current * time in mode1 + mode2 current * time in mode2 + sleep current * sleep time between wakeup)

This gives you the ratios of power used per mode, and how much power is used per active/sleep cycle so you can simply divide the total capacity by this cycle count consumption to work out the length


Replacement options 3-5 years after the devices lifetime, unless you pick a common or standardised size, it could be quite difficult to keep your device running, even if you yourself have no plan to replace the battery, leave the option, be it pads on the pcb or something similar, also could make sourcing problematic in future if you need to make more,
Depends heavily on the device, I've made water monitoring probes that are still active 6 years in off a single CR2320, though they are now in the part of the ESR curve where they are too weak to use there wireless daily download, and instead I force download them on nice warm day every 3 months or so, (smart software can work miracles here, I kept a tally on the cycles and the temperature, and every day it works out if its safe to transmit) 


Main point would be, if non-rechargable is your choice, make replacement possible, does not need to be easy, but possible, I ironically had the reverse of the normal issue, they don't make many waterproof and tiny enclosures, So mine is a normal IP54 enclosure with glue lines heatshrink fitted over the enclosure, to replace the batteries I would need to cut and peel it off, but its possible. If you glue or ultrasonic weld things, then its not really possib.e