You might save a couple kB in removing modules from the image. You're likely to have more benefit from building or assembling your own images with either the full build chain or the image builder. The packages that you add to the ROM remain compressed, and take up very roughly half of what they would if installed after flashing. As long as you've got three "erase blocks" free after you build the ROM (3 * 64 kB ~ 200 kB, as I recall), then you should be able to run the device and have it save config.
Yes, it can be "challenging" to resolve Internet issues without the Internet available! If you've got another device that can run OpenWrt, that might be a good way to get comfortable with the process. Knowing how to get the device into TFTP mode (and downloading a "release" version for TFTP that you can use), assuming it supports it, is a good thing to check before flashing your custom-built images. That way you can recover to a "known-good" image if you have to. A 3.3 V serial adapter is a good idea, but not essential for a device that supports TFTP in the boot loader.