I switched to AnySoftKeyboard and although the auto-correct/spellcheck is way worse (understandable since they're not collecting every word everyone's typing) the customization and terminal mode are great. I'd occasionally code on my phone in termux (the largest program written on that device was only around 2000 lines) and it did the job.
Is there a way to completely block them from accessing the network?
Your phone likely accepts a physical keyboard. I have a USB-C input, but can use a travel dongle (female USB-C device accepting USB-A) to attach.
I used this a few times to do some very light work when travelling. A good setup is picking up a cheap bluetooth keyboard/mouse combo and using the female input to get both. Many alternatives to this too, e.g. you can also attach a dock to your phone to get all devices your phone has the hardware to accept, and you'd be surprised what it does accept.