Dual-SIM phones: 1krBlocker checks messages arriving on both SIMs; conversations show a SIM badge so you can tell them apart. Messages you write are sent from the SIM Android uses for SMS: choose it in the phone's Settings → SIM cards (or Connections → SIM manager).
Message length: an SMS holds 160 characters (fewer, about 70, when it contains emoji or letters outside the basic alphabet, such as some Polish letters). Longer texts are sent as several SMS and joined again on the other phone. The counter in the message box shows "… left · … SMS": how many characters are left in the last part and how many SMS it takes. Your plan may count each part as one SMS.
A photo makes it an MMS, which needs mobile data.