This is basically two different questions:
- is residency allowed
- is naturalisation allowed
under your proposed conditions?
Residency
Can I quit my current work in Germany, but still stay here on Niederlassungserlaubnis without working at all for a long time period?
This would basically be possible under the condition that you are properly insured and you can support yourself during that time.
You should ask your present insurer what the costs would be to get an idea what it would entail.
Naturalisation
I have some savings which would be enough for me to live for the next 5-10 years. Therefore I don't want to use unemployment benefits either. I just want to stay here enough (5 more years) and apply for naturalisation at some point.
Based on the Implementation Instructions (Anwendungshinweise) for naturalization, your plan may fall foul due to §8(4) StAG
After working for 3 years (with contributions to the pension system), you will not be working for 5-10 years (with no contributions to the pension system).
They may come to the conclusion (at their discretion) that later in life you will not be able support yourself due to an insufficient old age pension.
Can I still get a citizenship without working, not paying tax, but also not receiving unemployment benefits?
As with all discretionary decisions, nobody can say for sure.
Not making contributions to the pension system, for a 5-10 year period, will probably reduce your chances.
Section 8 [Discretionary naturalization] StAG
(1) A foreigner who is legally ordinarily resident in Germany may be naturalized upon application if his or identity and nationality have been established and he or she
...
4. is able to support himself or herself and his or her dependants and
...
8.1.1.4 Zu Nummer 4 Unterhaltsfähigkeit
Der Einbürgerungsbewerber ist imstande, sich und seine Angehörigen zu ernähren, wenn er den eigenen und den Lebensunterhalt der Familie sowie etwaige gegen ihn gerichtete Unterhaltsansprüche nachhaltig und auf Dauer aus einem selbst erwirtschafteten Einkommen, einem eigenen Vermögen oder einem bestehenden Unterhaltsanspruch gegen einen Dritten bestreiten kann, ohne auf einen Anspruch auf Unterhalt aus öffentlichen Mitteln angewiesen zu sein (Unterhaltsfähigkeit). Bei verheirateten Einbürgerungsbewerbern ist es ausreichend, dass die Ehegatten hierzu gemeinsam in der Lage sind. Die Unterhaltsfähigkeit umfasst auch eine ausreichende soziale Absicherung gegen Krankheit, Pflegebedürftigkeit, Berufsoder Erwerbsunfähigkeit und für das Alter.
8.1.1.4 Re number 4, support ability
The naturalization applicant is able to support himself and his relatives if he can sustainably and permanently finance themselves and their family as well as any maintenance claims against him from his own income, his own assets or an existing maintenance claim against a third party, without being dependent on a claim to maintenance from public funds (maintenance ability). In the case of married naturalization applicants, it is sufficient that the spouses are jointly able to do so. The ability to support also includes sufficient social protection against illness, need for care, occupational or disability and for old age.
...
...
Der Bezug von Leistungen zur Sicherung des Lebensunterhalts (Arbeitslosengeld II und Sozialgeld) nach dem Zweiten Buch Sozialgesetzbuch) oder Leistungen nach dem Zwölften Buch Sozialgesetzbuch (Sozialhilfe) beziehungsweise das Bestehen eines entsprechenden Anspruchs steht der Einbürgerung entgegen. (vergleiche Nummer 10.1.1.3). Dies gilt auch dann, wenn der Einbürgerungsbewerber den Umstand, der ihn zur Inanspruchnahme dieser Leistungen berechtigt, nicht zu vertreten hat.
Obtaining benefits to secure a livelihood (unemployment benefit II and social benefits) according to Book Two of the Social Code) or benefits according to Book Twelve of the Social Code (social assistance) or the existence of a corresponding entitlement is contrary to naturalization. (compare number 10.1.1.3). This also applies if the naturalization applicant is not responsible for the circumstance that entitles him to use these services.
...
Note:
This does not apply to recipients of unemployment benefit I (i.e. normal unemployment benefits), Book Three of the Social Code (Arbeitslosengeld [I] Drittes Buch Sozialgesetzbuch – SGB III).
Sources:
