I've always been scared of backlogs. I've rarely seen one that doesn't have hundreds of issues, spawning years of technical debt. They usually come bundled with a team that's already stretched thin.
When accessibility backlogs are that intimidating, nothing gets prioritised properly. The backlog grows further and nothing ever changes. It's always same shit, different day.
Accessibility feels overwhelming because the full spec is enormous.
But you don't need to fix everything. You need to fix one thing.
Pick something small enough you can do in a few days, just as long as it matters. If you don't know where to start, I can bet your forms have problems. No labels, errors or hints not associated with the controls, error checking isn't announced by a screen reader, missing focus outlines. Take your pick.
Scope it, prioritise it, do it and ship it.
I can guarantee that one fix matters to someone right now.
And it also matters internally because you now have evidence that your team ships without accessibility blocking ongoing work. That makes the next ticket easier to justify. And the one after that.
The best thing about it is you don't need a roadmap. You just need a ticket to get the ball rolling.