Depending on their age etc you could leave them in place. Being a galaxy they will currently be wired eol 1k 1k. Need to check your new panel will support that or you may need to chnage the jumpers/resistors inside the detectors.
Ion is a good panel but iirc the wireless side is 1 way (but im not sure never fitted one)
You can diy anything but im sure you have seen diy in your profession. Some or ok some are terrible
Be cautious with this. All apps running cost is either funded by product sales or fees. An alarm system notification cloud is a more critical system than say a lighting app or heating etc. If it goes down for a week it doesnt really matter. On an intruder alarm systm it matters a lot. Also free systems tend to be obsoleted quicker than fee based ones as additional income is required to fund it forcing new equipment to be installed and mroe ewaste. This is common in the home automation sector.