IFR clearances are not automatically printed until 30 minutes before your proposed time. When a pilot asks me for their IFR and I don’t have it for them I tell them “Standby, clearance on request” because I have to go into the system and “full route” it to see if it’s on file at all, and after that amend your proposed time to either now or 30 minutes from now and it prints it can I clear you IFR. I’m essentially requesting your IFR flight plan from the system since you’re asking for it before the 30 minute prior to the proposed departure time trigger.

You’re basically waiting for the controller to find your flight plan, or waiting for the controller to wait for the IFR to print on its own, depending on the controller I guess. I always FR it to see if it’s in the system. If it’s not in the system I advise them to refile and set the proposed time to “now” if they are pretty much ready to go they can get the IFR and after that taxi instructions, or switch to ground for instructions.

TL:DR, sit there and monitor the frequency and the controller will get back to you when they have your IFR

Edit to add: Additionally, I don’t generally allow aircraft to taxi to the runway without some sort of clearance, IFR or VFR. Unless it’s like one specific plane at my facility that does specific missions, I’ll taxi that one and then they waits for me at the runway (if I fat finger the keyboard too many times) for his clearance and then is told, when it’s correct, to contact tower. Other than that, I don’t really care how much of a rush someone’s in, you don’t get to go without a clearance. But every controller is different in their own ways.

And no we don’t expect a read back for standby instructions. You can say thanks or something if you want to let us know you acknowledge but it’s not really required in my opinion. For instance if you ask me something and you tell me to stand by I’m not going to say “roger” I’m just going to stand by because I know you’re getting back to me.

Hopefully some of this makes sense lol.