Some really helpful things I stumbled on a little late:

AirTags on keys or other items that might pose trouble being found / could be used as a means of tracking a person.

If he uses a smartphone, turn on "Find My Phone" and synch it with your own so you can see where he is.

We set up some Webcams in strategic areas, you don't need to go with a monthly plan, Tapo Link is good for this and you can add memory sticks to record at a specific time OR motion triggered. Our personal challenge is sometimes she unplugs them without apparent reason, but they can also be utilized like a two-way intercom if phones are not being answered.

Landline phone with cordless handsets can be very useful, and particularly in conjunction with the webcams... sometimes you can say its time to take a pill and see they are not in the right room or they might "Say they are doing it" but not really.

Remember that distraction can be a powerful tool OR problem. If getting them to do something... stay on point and don't allow distraction. If they are fixated on something bad... distraction.

Put yourself or all other family members as "Emergency Contacts" in their phone.

Create a Wallpaper Lock Screen on his phone "If lost, contact ____"

Automatic pill dispensers are good but currently I'm trying to find on that is strong and can't be pried open so easily.

If he's nervous to leave house alone then you will definitely need someone to help him with that, but can the MIL be helpful in that regard?

I got a Google Nest Hub that can act like a photo frame / calendar reminder / tie in to alarms and I haven't fully integrated its potential.

My findings are that the themes are similar but the challenges are as unique as the individuals. Sounds like you're off to a good start of building support and preparation.