That depends entirely upon what you want to do to it.
I am also not sure what tools will exist as is for the game -- Shin Megami Tensei is a popular franchise but not one that is hacked all that often compared to Mario, Sonic, pokemon, various Square (Enix) titles, megaman and the rest of the usual suspects list. If indeed it is unreal engine then that can provide some options over complete custom engines as by their nature engines do provide something for devs to use so most do actually use it, though plenty add a lot of custom stuff on top of things.
Anyway this is all getting a bit abstract.
There are a few things done
https://gbatemp.net/threads/shin-megami-tensei-v-60-fps-graphics-mods.602852/ https://gbatemp.net/threads/shin-megami-tensei-v-discussion-save-editor-soon.602923/ but neither of those are particularly far down the path that things like pokemon tread where there are nice programs you can click things in to edit other aspects.
The Switch, being a modern device, uses file systems to house files. This means you can break the ROM up into whatever files the dev left it as. Maybe the unreal engine being what it is allows you go guess the nature of some of the formats, maybe they are shared with (or at least inspired by/derived from) earlier games in the franchise, maybe the names/sizes/directory names/extensions/magic stamps/surrounding files give context as to what they contain.
https://github.com/jakcron/nstool is a reasonable start there but there are others.
Many will want to manipulate the memory --
cheats are quite useful when figuring things out, as is the ability to view the memory. There are some emulators with minor debugging options but most will probably go for on hardware debugging via network connection
https://gbatemp.net/threads/atmospheres-gdb.602435/
Whatever you use for
cheats in your given custom firmware will do for memory things too, though current emulators have an easier time than they do for more intense debugging.
Graphics. The Switch, being a 3d based effort, does not have as much in the way of 2d options as earlier consoles and you will instead probably find yourself looking at models and textures more than classic 2d stuff in a tile editor/tile viewer (though textures are mostly still 2d affairs and some 2d stuff for things like fonts will exist).
3d games also tend to mean 3d models for levels, and files to handle that as well.
Audio
Might as well start with
https://www.vgmstream.org/
More for ripping/playback than editing but still good stuff. Editing can also be various things from tweaking existing stuff within its own parameters to adding whole new tracks.
Text. This is probably still going to be in the realm of general tools rather than needing something more modern.
For English to English stuff then relative search is a good start
http://www.romhacking.net/utilities/513/
I cover a lot more on text hacking in
https://gbatemp.net/threads/gbatemp-rom-hacking-documentation-project-new-2016-edition-out.73394/ , might also do for general hacking and how that plays out.
I don't know what we are suggesting for Switch compression these days -- Nintendo usually provides their own format (though devs can use another) and that means tools to handle it get made and disseminated among hackers.
