background changing

One very handy way to do this is to have your game logic independent from your NMI routine.

Inside your NMI routine, perform 'vital' functions such as sprite DMA, palette updates and VRAM I/O, controller reading, and audio playback, and at the very end, increment some variable named 'nmicount'.

Elsewhere in your code, you will have a set of routines which start by waiting for nmicount to become nonzero (and then zeroing it afterwards), then perform some functions (checking input, executing game logic, queueing background/sprite updates, etc.), and then JMPing to some other routine based on the outcome (possibly the same one it's already in). Your very first routine in this group will be called 'Titlescreen' (or equivalent) and you will JMP to it at the end of your reset code (after NMIs are enabled, of course).

This has the advantage of being very easy to organize and expand and having less clutter in the 'important' routines (namely, your NMI and IRQ handlers).

I'm using this exact method in a game I'm making which is ALSO quite similar to Pac-Man but is still very much different. The player is a Drip™ of water, each level is a set of pipes, and the player must rust them while being chased by 4 enemies and avoiding hazards (electric coils between the pipes, projectiles shooting from the pipes) and collecting bonuses. The game itself is basically a port/remake of the Amiga game "Drip" by Art Skiles.