As per the above any app that is free will be the least reliable. While downtime for heating control, smart lights etc its far from ideal for a security system.
6 do your kids not watch telly on one, rather than use the huge screen we would all rather watch.
Mine use small screens for things i cant get, but thats the way its going and im 44 (this month ffs) so im proper old
Timers and a vcr or knowing what time your program will start. We all want automation done right. Back to basics it's knowing someone is in an area they shouldn't be. Ie movement detector and an armed system.
I like apps, however I know they all need support.
I know its not what the OP wants we all want something good for free, but id like at things like availability (ie how much its up) before I looked at cost. I can give anyone an app for free, god knows how reliable it will be though.
Bear in mind the pstn shut off will have unknown issues with comms. I wouldnt be using pstn for any primary path.
However the question is very open, I like HKC for rf, 6wb like texecom, gg likes honeywell etc
If you got it purely on rf strength you are way wrong even pyronix has more tx power than risco (it's near the bottom) even visonic has more tx power.
Using what you have I get but if it's steam powered it's probably best to replace. I used to rate the 1 way scantronic 4xxx stuff but that was in the 90s rf is way different now