I did a few quotes and I identified the possibility of flying back First within budget. Plus the Marriott 40 points, I could get 700 points total:
ZRH-SIN J 200
SIN-CGK J 40
CGK-SIN Y 20
SIN-ZRH F 400
Marriott 40

LCA - TLV?

1 hour flight.

which *A carrier is flying that?

Aegean.

(Seems irrelevant for getting to CGK, though)

Issue is TLV is "Middle East and Asia" so it wouldn't help with the Jakarta route but it's good to keep in mind for mile runs.

TLV is the points run sweetspot since it's pointed as intercontinental, but priced as Europe. Another good one is Amman.

The cheapest ET business class fare is not eligible to earn miles thus I suspect it will not earn any points as well.

And BEY, Beirut airport is quite impressive

JNB-ZRH-DBX in F is pretty good pricewise and QPwise. 800 QP for around 4000 EUR in luxury. And from DBX it is easy to add an DBX-CAI-AMM in C for another 800 points and another 1000 EUR

Is at one of those sweetspot destinations a direct turnaround possible?

Any recent experiences with cheap rates? ZRH-TLV-ZRH seems to be CHF >1.5k almost all the time�

Those living near or in a hub will usually be worse off in Miles and More and for pricing but your best bet is going to the Balkans (Ljubljana, Split, Dubrovnik, Tirana etc.) or Athens and then connecting once or twice via Munich, Geneva or Vienna and then onwards to Frankfurt, Zurich or Brussels then further on to Tel Aviv for around 5xx to 6xx €.
Similarly, Beirut pops up from time to time with a decent fare when it is being run (mind the visa on arrival if you do enter the country, it is very cheap however)

Finally, slightly more expensive but safer is Amman - mind the visa on arrival for Jordan compared to Israel's ETA, this one is a little pricey - with Split and Athens the best starting points here and consistently able to show fares with 2 connections in the 6xx € range.

Another option for you is to go during the Winter Schedule or in the Spring (only for UAE) to the Middle East with Eurowings from either Berlin, Hamburg, D�sseldorf, Cologne, Hanover, Nuremberg, Prague, Stockholm or in your case Stuttgart.
If you can find a cheap way to position yourself to any of these airports, there are fares for even direct turnarounds in the high 4xx to 6xx range for 400 P/QP/HON depending on the dates and how far ahead you are booking, they are great to start the year or garner the final remaining points necessary.

As a Swiss citizen, Lebanon, Saudi Arabia and Jordan are all Visa on Arrival or eVisa, ETA for Israel and visa free for UAE, same as Germany.
Keep that in mind for added costs.
Israel and Lebanon are cheap while Jordan is decently pricey and Saudi is very expensive for a visa.

As long as the spend doesn't exceed 3 € / CHF per QP/HON it is an okay deal usually but anything intercontinental you can keep under 2 is fantastic.

Edit: Pretty much in all sweetspot destinations in the Middle East a direct turnaround can be done though not without some stress.

True, but depends on what your after. You can directly renew your SEN by simply paying for it, and that'll be less than 4000 EUR. HON is, of course, another story.

woudn't it be 1600 QP now? for a roundtrip it should be 400 + 400 + 400 + 400
but I'm unable to find any availability for that price, only for about ~4100� and ZRH/FRA - DXB is in C instead of F, so less QP

I remember seeing some nice bargains some time ago for ZHT-ZRH-TLV-ATH-TLV-ZRH-ZHT in C for about €600 at the time, which would have gave you 880 QP‘s.

Of course with the new war this is maybe not the best time to check/ask, but I had noticed that in the last months first the additional train ride from ZHT was not offered anymore, and then the return leg via TLV also not anymore.

Does anyone have insight on this?

Just to correct you, LJU has never been and is not in the Balcans, it is Central Europe, the Balcans start south of the Sava river, so a little lower than ZAG. And no, LJU (SLO) has never been Eastern Europe either.
Regarding your proposal for LJU/SPU/DBV/TIA those cities are all quite expensive to fly to, so is TRS, speaking of LH Group and other *A carriers, ZAG has sometimes decent fares, but is codeshare with OS, which is not appealing on thei DHC prop airplanes. Besides To/from LJU/TRS and KLU is quite difficult to get the cheapest fares, and those are limited from W onwards, and Z in business.