I’ve no idea how long the interview process will take to book – my experience in March this year I rang on the Thursday, had a virtual interview on Sunday and my application was approved an hour later. The good news for you is the timeline looks similar to mine, it looks like you’re on the digital route so the interview should be the last thing and your status should go to approved from the docs received status – skipping out the dreaded processing update as mine did. So keep trying to book that appointment – I suppose you’ll just have to be patient , try as much as you can to get through. Good luck.