Well, I followed your advice and I managed to make some progress, though not for long before hitting a road block. Turns out, certain sections of the maze are made out of what I call "tile groups." Basically a "tile group" is multiple consecutive copies of the same tile, assigned as a group to one single value in the level data, similarly to how singular tiles are. I can't seem to get around these "tile group" values to try adding values for singular non-grouped tiles in their place, or it'll corrupt my level; basically not being able to work around this makes the level far less customizable than I originally anticipated.
"Tile groups" for walls and dots in this image are highlighted in red. Though there appears to be less due to how they connect, there are actually twelve groups of four in this cropped portion of the level alone.
So with that in mind, how far did I make it with my custom level? ...Not very far.
By the time I made it to the lower half of the screen, I realized that there were even more variants of tile groups all scattered throughout the maze, including groups of three. My level looked nothing how I imagined it would, and I realized it wasn't going to for as long as there's no workaround for modifying all these tile groups.
Good news is, I was able to get a lot of notes down regarding how the levels in Pac-Man NES appear to be made. Including notes on the values of most of used level tiles, the location of the level data and some other things. I don't know if I'll continue to try level hacking, at this rate it all depends on those tile groups getting in the way, but for now I'm sure my notes may come in handy for someone in the future.
Here's all the notes I took throughout my attempts so far, Pastebin link. Even if I end up scrapping the whole "custom level" idea I had in mind for Pac-Man NES Improvement, this did prove to be a decent exercise for an aspect of hacking I've never even touched before. https://pastebin.com/zmyXDSgS