How to answer dates/day questions for multiple entry Schengen Tourist Visa application, with unplanned dates?

I have a confirmed & all-booked 9 day trip to Germany in less than 3 months, & a one more trip few (~6) months after (it will be certainly Schengen Member State, but dates/days/country/city will depend on flight-prices/my-leave-schedule)).

I came upon this question which asked almost same as me, but the main difference is that That OP has fixed dates of his 2nd or further trips, whereas I do not have.

So, how should I answer these questions in visa application form? I am going to tick Multiple Entry. FWIW, I have 4 Schengen Tourist Visas in last 2 years from 4 different member states, three single entry and one multiple entries, all expired as of now, all spaced ~6 months apart.

1. Date of Departure from Schengen? I do not know the exact date of my second trip.

2. Number of Days? First trip is 9 days, second trip will also be ~9 days (depends on flight price) Should I write 18?

3. Main destination? First trip its Germany, but not decided for second trip

EDIT: Bringing my comment here: Found one more related question, but the answer says Field 30 should have the date of last departure. I do not have this, although I am planning to attach a carefully crafted cover letter explaining my intentions/plans. That Question's OP commented on answer that he will book a refundable flight for future trips, but I cannot.

EDIT2: Update: So I asked Single entry again, as standard, & because only first trip's details were confirmed, applied, and got Multiple Entry 30Days/3 Months Visa..