I'm in my early 40s (EU-citizen) and finally started investing thanks to retirejapan.

I am about to set up my Tsumitate Nisa.
Now I am wondering if I should enroll in Ideco as well?
There are two points that keep my from opening an Ideco account:
1. I am a civil servant and the max. amount I can invest is 12.000円 monthly. Is that even worth it or should I rather use that money to max. out Nisa?
2. I am not planning to spend my retirement in Japan, planning to go back to my home country. So far it looks like I will stay here until 60, but it could also be possible that I'll leave in 10-15 years. I am aware that I am not able to withdraw my money until the age of 60.
Should I split into:
* 12.000円 Ideco
* 18.000円 Tsumitate Nisa
Or should I invest all 30.000円 in Tsumitate Nisa?
Are there any advantages/disadvantages?
Another option is to open an IB account instead of Ideco. Then I would invest a bigger lump sum and just leave it. However, I still need to do some research about taxes etc.
Thank you for your advice!
Best regards,
Ichigo