...because now it's impossible to set planet-specific recipies in assembling machines.

For example, I can't craft Gleba's science in biochamber, while in sandbox.

I don't understand your question, if it's a question, or what you are trying to say. This is a feature, so it is possible to do what you said is impossible.

The problem I got with the mod is... Ok, step by step:
- I'm on Gleba.
- I press Shift+B to enter the sandbox mode.
- I build a facility setup for Gleba's science.
- When I set recipes for Biochambers, the icons of recipes on Biochambers' entities (in alt-mode) show me those recipes with a red crossed circle on them, and Biochambers (when I hover a mouse above them) say it's not relevant surface they are placed on.

So it looks like sandbox surface simulates Nauvis's conditions: gravity, pressure, etc.
But there are many resipes in Factirio 2.0 which can be executed only on proper planet (surface with proper conditions).

My suggestion/ask is to let the user set surface properties.
Maybe not separately (gravity, pressure...) — give us a button(-s) to make sandbox surface simulate another planet's conditions.

Could I make it more clear?

You can already do this. If you look at the images and features under the information tab on this portal, or at the tips/tricks within the game, or by curiously hovering over the few elements on the GUI, you'll see what I mean.

Oh... Really. =)
I installed the mod from the in-game UI. There are neither images nor description. And looks like I haven't been enough curious in hovering over the UI. And you've got somethingtohide in the tips/tricks within the game. =)
Thanks for the feedback! Mod is cool.