Pi-hole should be the sole DNS server for your clients (no others or ads will leak through)!
So yes if Pi-hole goes down, you'll have to reconfigure your router LAN DHCP DNS settings to restore Internet connectivity.
You'd also have to let the clients renew their DHCP leases to receive the newly configured DNS server(s).
Or if have set DNS on that Debian laptop manually, you'd have to configure another DNS server(s).
I have two Pi-hole nodes on two different Raspberry Pi's for my home network to prevent disruption when one goes down.
For example when goes down while upgrading, tweaking or making mistakes.
Heck I've even got five Pi-hole instances bc I'm also experimenting/learning a bit:
one "primary" on a Pi 1B;
one "secondary" on a Pi 1B+;
one running in Docker with stable v5 on my NAS;
one running in Docker with beta v6 on my NAS;
and one running in a VM with beta v6 on an old Intel NUC that I turned into a hypervisor;
There are many options to create a second Pi-hole node for redundancy.