Hey there! :)
I've seen your cool mod many times, but only just now got around to looking into it. The biggest draw for me is the Factorissimo integration you implemented.

I haven't installed the mod yet, but am I correct in thinking there are no tunnels for the roads? Because that would be the biggest drawback I'd have with the mod.
Especially since you've implemented channel segregation and cross-surface support, allowing your drones to enter and leave Factorissimo halls, I'm surprised that tunnels (with a similar functionality) aren't implemented.
Another, much simpler idea would be: Drone enters the tunnel entrance -> tunnel length and drone speed are calculated together -> drone spawns at the tunnel exit.

What do you think of it?

That is a pretty good logical next step.

I installed your mod and tested it thoroughly. Yep, we absolutely need tunnels! :D

I'd probably look into adding this as a feature after I do 2.1.

Please try to make this feature also possible for 2.0.

Some feedback on that idea?

The idea: drones drive into one entrance, stay hidden for a travel time, and come out at the other end. The two ends' roads join into one network, the same way the Factorissimo door joins a factory's inside and outside networks.

How tunnels connect: this is left open with three options:
Direct pairs, like underground belts.
Channels, where entrances on the same channel connect.
An underground network, where a drone can enter at any entrance and leave at any other.

Balancing: a maximum distance (possibly in tiers, like underground belts), a travel time of roughly distance divided by a tunnel speed, and an open question about whether to limit how many drones can be inside at once.

That's what I have so far at the drawing board.

"An underground network, where a drone can enter at any entrance and leave at any other."

That was also my initial idea, and it would offer a huge number of possibilities and differentiate it even further from logistics bots. But this would certainly be the most complex implementation.
One could then create a kind of underground level (similar to what "Beltlayer by TeuLux" does. This mod, incidentally, is also compatible with Factorissimo and creates a unique underground floor for each Factorissimo hall.)

But for the sake of simplicity, one can also take my first idea, that a drone drives into the tunnel entrance, despawns, time passes (tunnel length is calculated with drone speed) and then the drone reappears at the exit.

I generally think tunnels of different lengths are a good thing. Since you have two different roads with different speed, you have to choose one of the two speeds when going through the tunnels.