Appointments - One service provider, two different services

Hi guys

I know this issue seems to come up multiple times throughout the forum, yet i still haven't been able to figure it out, despite trying many things.

the need:
a therapist offers therapy sessions. the first session is 150 minutes(takes a bit longer because of getting to know the customer), 2nd and 3rd appointments only take 120 minutes.

the current status:
time base is set to 30 minutes. service 1 is set to 150 minutes, service 2 is set to 120 minutes.

the test:
saturday working hours are 9 - 6pm. the day is completely empty, so no appointments booked yet. the first customer , let's call him Fred, books a 2nd therapy at 11am. this works perfectly and blocks the time from 11am - 1pm.

the problem:
second customer, let's call him Ralf, comes along and wants to book a first appointment(150 minutes). the next available time slot is shown at 2pm. this is not good, since Freds appointment only goes to 1pm. the therapist loses 1 complete hour.

the (partial) solution:
i activated the "durations add-on". this resolves the above issue.

the next problem:
now Ralf is shown time slots every 30 minutes. this is not ideal, because if he decides to book at 1.30, the therapist basically again sits around for 30 minutes and loses this time.

i really went through the entire forum, tried functions.php changes etc, so far nothing has worked.

would really appreciate any help to resolve this issue. thanks in advance for an awesome plugin and hopefully a solution!