If you have one or two friends who are already on signal, contact them through that. Sometimes they won’t reply as quickly, but that’s fine, and you might need to learn to live with that. You can also use whatsapp less often (eg disable notifications, only use it on a VM on your PC), and make it clear (in your about, for example) that you’re easier to reach via signal.
You can also use the network effect to get more people to contact you through the app after you have a few. Let’s assume you talk to friends A and B on signal, and you have a group with A, B, and C. You could simply leave the group, and say that you’d rather have this conversation happen on a more private platform like signal. It’s usually not a hassle for most people to just install another app to contact you, and from my experience, C will usually join signal after this.
This might not work for you, and you might see a reason to keep using WhatsApp for most of your chats, but I hope this helps nevertheless.