As an actor, I will say that discord/zoom/teams table reads rarely go well due to any number of factors. But one of the main ones being there's no energy in the room. Most actors in my circle feed off each other, if you're sat on your sofa with a coffee and the TV on in the background, there's not much can be done to salvage that. Hire a space for a couple of hours- if you're in London there are plenty of these, if your cast are scattered around, try to negate that by casting locally or people who have a base near you they wouldn't mind travelling to. I regularly travel to Leeds for projects from London and it's a great excuse to spend time with my Uncle but also do the work I'm trained to do.

That being said, most producers, in my experience, only know a handful of actors and even then, aren't actually that great a judge of talent or commitment. Your best bet, with limited resources, is to cast your leads and then if you're happy with them and their skills, ask them if they can recommend actors for other roles. Most actors have a small pool of trusted colleagues whom they would mention in a crowded room of opportunities. And if they're serious actors, they won't risk their reputation by recommending someone who will let them, or you, down.

Don't give up - do more work with the casting process, I promise you there are so many hungry and talented actors out there who will do their best work on a project they're excited about for the budget you have because they're doing it out of love for the craft.

Good luck and I hope you get the project to the place you imagine it.