ᐅ Separate contracting for windows and roller shutters – the hardware manufacturer is blocking progress!

We are building with a local general contractor (GC).
When signing the contract with the GC (it had to be done quickly on our side due to the bank’s requirements), we secured a contractual agreement allowing us to remove individual trades from the contract or add items without additional costs up until the building permit / planning permission submission.

Over the past few weeks, we have used this time to obtain comparative quotes.

The only trade we would like to remove are the overpriced electric aluminum roller shutters (9 units for almost €15,000). Since we cannot / are not allowed to remove them individually, we would have to cancel the entire package (windows, roller shutters, exterior doors, and exterior window sills).
The refund for this would be:
€16,600 base price for windows and doors
+ €2,650 (foil-coated windows)
+ €480 (mullion profiles)
+ €2,020 (side entrance door)
+ €8,660 (roller shutter installation)
+ €4,070 (electric roller shutter drives)
+ €2,580 (aluminum roller shutters)
= total reimbursement of €37,060

Based on the scaled plans, we obtained comparative offers and received a very good quote:
Price for everything around €34,000 (with the following added benefits compared to the GC’s offer:
- 14/14 roller shutters instead of 9/14 roller shutters,
- sliding lift-and-slide door instead of standard patio doors,
- app control for the roller shutters instead of no app control.

Everything else is included.

However, these would be Salamander windows (separate contract) instead of Rehau windows (GC). Since the U-value is at least equivalent and I can’t find anything negative about these windows, we don’t mind.

Now our problem:
Our GC, despite multiple requests, will not provide a detailed specification (bill of quantities) for the windows and also refuses to give us the exact dimensions.
Their explanation is that the GC sends the building permit / planning permission plans to the trades (window manufacturers), who then submit their quotes. All openings are dimensioned in these plans. The process wouldn’t be changed just for our request.

However, we must decide whether or not to award the window contract separately before the building permit / planning permission submission.

Is this procedure normal?

Another concern for us is that the GC says that we could potentially lose the construction time guarantee and fixed price guarantee if our own work causes delays in the construction schedule, which is somewhat understandable.
It’s now December, and construction is planned to start in March 2023 (the building permit / planning permission should presumably be submitted this year).
Is there definitely enough time for the window manufacturer to have the windows ready on time to avoid delays?
What are typical delivery times?
Does anyone have experience with this?

Small additional question:
We have a quote for a photovoltaic system. The supplier advises us to remove the photovoltaic preparation from the GC’s contract because it is very expensive at about €1,900. They recommend approaching the construction workers directly to see if they can install an empty conduit for a few euros. They also said that nowadays, new builds usually have an enlarged electrical panel / breaker box, or you should talk directly to the electrician if needed.
The photovoltaic supplier would, in any case, always find a way to get the cables into the utility room and connect the system.
What do you think?

I would appreciate any help!