I need to upgrade my Pixel 2 to a dual sim phone too. I think this seems like the easiest way. Not sure how WhatsApp etc support dual sim though.
But you can always at the very least set up the accounts, using your browser on a PC.
You just won't get chat notifications for multiple numbers on one device.
Telegram app lets you have 3 accounts logged in, the only similarity being that setting up a Telegram account requires phone number authentication. But the similarity ends because a Telegram account on your device doesn't need to be linked with a phone number accessible from your device.
Few people have Telegram, Signal seems like a very niche tech/journalist thing.
I'm not really focusing on the dating use case and have already solved that for myself, and for you, and I'm not sure why you are overthinking it aside from your upcoming expensive of buying a new phone and additional cellular plan. The practicality is already there.
I’m aware of people that immediately ask for social media, and rationalize it with trying to disprove infidelity or an assurance they wont get murdered, I can tell you that most people I’ve dated over the past two years did not ask for that. At this point I think there is a level of maturity involved in not asking questions you don't really want the answer to, as they too would prefer not to lead with their social media.
Its pretty much the same as introducing a date to your friends. You don't typically lead with that. YMMV.
Though I think a lot of people do want to see IG, just to get an idea about your life, friends etc. Like a way to know about you without prodding too much. I anyway don't use IG anymore so I just say I don't have it, and as you said they seem to be fine with it.