> the diodes make up the difference

That's important. 5 years is a big ask for a battery supply.

Anything at all that can be got rid of or not used in the first
place will help get you to that 5 years

There is no particular reason to always use a PIC at 5V. As
you know, they will function quite happily down to 3V and the
LF627/628 down to 2V. And 2V is a pretty knackered Lithium

There are probably 5V devices hanging off the PIC, and that
can be overcome. Maybe they can have their supply, and give
the PIC its own battery

If you have any more details Paul - current, sleep, circuit etc -
that would be helpful

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