For the patches, here's the ones I'd say are essential and widely useful:
- Custom font sizes -- much nicer and more useful range of font sizes, especially on badly formatted books
- 24 line spacing values -- same idea, but even more useful in some ways
- Header/footer, page space, margin patches -- uniform narrow margins for aesthetics and maximizing screen space
- SmartLink patches -- make the bottom buttons on the homescreen actually do something you find useful
- Uniform font sizes -- you'll get frustrated by the font size discrepancy quite quickly if you switch between epub and kepub
- Increase page navigation history -- very useful for reference material and large fiction books
Apart from development stuff and launching external applications, the main thing I use NickelMenu for is inverted rotation (so I can hold my clara upside down by the thick part), dark mode toggle, usb connection, starting a FTP server for file transfer, and some custom selection menu lookup/notetaking stuff.
If you sideload books, the kepub reader is much faster and better IMO. You can use calibre KTE or kepubify for this.
If your sideloaded books have series metadata, my NickelSeries mod is a must.
Some people like having a clock while reading; NickelClock is the cleanest way to do this.
Many people with bluetooth Kobos seem to like the kobo-btpt mod.
Some fonts I like are: Equity, Charis SIL, Gentium Plus, Minion Pro, Caladea, Source Serif 4, Roboto, and Roboto Serif.