Best calenders for sales teams?

We run a B2B sales outsourcing agency. My cofounder is essentially the closer for everything – he handles closing deals for our clients and closing new clients for us. We're seeing a big increase in meeting volume and right now we literally just text each other "hey are you free at 2?" which obviously doesn't scale.

Here's what makes this tricky:

Problem 1 – We need a shared booking view for our sales team

Our salespeople are setting appointments for my cofounder to close. Right now there's too much back and forth. We need some kind of calendar or scheduling link our team can see and just book directly onto his calendar without having to ask.

Problem 2 – He has a ton of email accounts

On top of our main business, he does independent consulting on the side. He's got about 3 Google accounts and 3 Outlook accounts that all have their own calendars. Any tool we use needs to pull availability from ALL of those so nothing gets double booked. This is the part that feels like a nightmare.

Problem 3 – Variable meeting lengths

Not every meeting is the same. For the deals he closes on behalf of our clients, it starts as a ~15 minute phone call. If the prospect is interested, it turns into a ~30 minute team meeting. For closing new clients for our own agency, those are around 30 minutes. So we need some flexibility in meeting types/durations.

Has anyone dealt with something like this? What tool or combo of tools would you recommend? We've looked at Calendly and Cal dot com but not sure if they handle the multi-account syncing well. Open to any suggestions.

Thanks in advance.