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

Question

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..

Answer 1

There is no easy way to enter this because that's not what multiple-entry visas are for. Article 24 of the Schengen visa code reads:

2. Without prejudice to Article 12(a), multiple-entry visas shall be issued with a period of validity between six months and five years, where the following conditions are met:

   (a) the applicant proves the need or justifies the intention to travel frequently and/or regularly, in particular due to his occupational or family status, such as business persons, civil servants engaged in regular official contacts with Member States and EU institutions, representatives of civil society organisations travelling for the purpose of educational training, seminars and conferences, family members of citizens of the Union, family members of third-country nationals legally residing in Member States and seafarers; and

   (b) the applicant proves his integrity and reliability, in particular the lawful use of previous uniform visas or visas with limited territorial validity, his economic situation in the country of origin and his genuine intention to leave the territory of the Member States before the expiry of the visa applied for.

Tourism is not mentioned there, nor are trips to multiple countries. Strictly speaking multiple-entry visas are for repeated trip to one specific country, not a way for one country in the Schengen area to authorise a completely unrelated trip to another country in the area (even if incidental visits certainly are allowed).

In theory, you are therefore supposed to apply for a new visa every time. And the plausibility of your plans certainly factor in the evaluation, so some unspecified trip at an unspecified date is not a very convincing reason to get a multiple-entry visa. Basically, it's just a way to say "I don't want to answer all your questions again, please let me do what I want", which is understandable but obviously not something the people asking the questions are likely to see with a kind eye.

Beyond that, multiple-entry visas are also a way for consulate to save themselves some work and you can certainly be lucky and get one (as you already did on one of your previous trips apparently). So you can certainly check the box and join a letter to make your case. But there is no guarantee that you will get what you want.