I'm aware of the link, but people don't have Signal, everyone has WhatsApp.
In the UK at least, a recent ofcom ( Communications regulator ) report suggested that 76% of people reported using WhatsApp in the previous 3 months.
This is close enough to the 83% of people who reported making a phone call in the past 3 months that you can consider "everyone" to have WhatsApp in the same way you'd consider everyone can make phone calls. Yes, there are notable exceptions you may have to accommodate for or be prepared for if necessary, but you can by default assume everyone has it on their phone.
If OP's objection is to Meta, then of course don't use WhatsApp. But if the objection is Facebook as a platform then a message group may be suitable.
