Municipal governance is a tricky balance of public wants/needs against a litany of constraints. I am not sure how this will compare to your previous role. Last May I ordered 2 bike repair stands to have in place for June bike Month. One is in service, the other is in a packing crate in the basement of a building waiting for approvals to clear.
As a "Chief of staff" your role will likely involve a lot of chasing - chasing staff to follow up on everything from the status of an All-way stop review to why syringes are showing up in parks, to how to deal with seniors that can't shovel sidewalks. Your boss will be riding the high of the post-election honeymoon for the year you are with him, and you will be leaving just as the shine starts to tarnish. The timing of your departure may be tricky for the riding.
A chief of staff role is often a grooming position - one that puts you in touch with all the players of the political spectrum - from the activists to investors to legislators to commissioners and senior leadership. Leaving his office, you would normally either be tuned in to politics in a way that makes it hard to exit, or in a way that makes you want out NOW.
How much of a wordsmith are you? How good are you at finding the positive spin to problems? How in-tune are you to saying the right thing at the right time to the right people without offending the other folks in the room? If you are leaving in a year, do you care? Honestly? If there is a city hall scandal or success, and a reporter aims a camera at you, how comfortable are you going on air to the biggest market in Canada? How comfortable are you backing up a decision by city hall that goes against your recommendations?
I suspect that in this role you will be spending most of your day on the phone with constituents and staff trying to make things right by both sides. Sometimes you have to give hard "no's" and sometimes you give soft yeses. Both of these can be difficult to deliver.
As it affects your FIRE plans, I would go for it if you are up to the role. Its insurance money that you should never need, but will give you the freedom to chase bigger dreams. I don't think this gets in the way of short term goals. I don't know that though - does it? But make it very clear that you are bridging for the next person and will not be there long term. otherwise, next election you may find yourself a running-mate or campaign organizer. Its kindof how City Hall works.