You can use just about anything but the best idea - for whatever method you use - is to have a code language or set of symbols so that even if someone is spying on you and can see what you’re entering in a text, they won’t be able to make sense of it. (I always like the simplest field tactics.)
For example, you could pick a different subject every day such as “next weekend’s block party” and just enter in a bunch of random gobbledygook about who’s going to bring the salad and so on, but the accompany this chitchat with emoticons or turns of phrase that describe the scene. Three pumpkins in a row with a series of numbers “three unknown subjects jogging seen on this block” and so on.
What kind of security concerns are there? What about the encryption of Signal?
I can see legal issues when discussing named individuals but figure that I could switch someone to emails or personal phone calls if in-depth discussions of a criminal neighbor were required.