I-751 can technically take 2-3 years to get approved. There's a reason why USCIS issues 48 month extension letters as oppose to 12, 18 or 24 months like they used to.
Don't apply for naturalisation, this is a British or Aussie term. This country uses naturalization.
If you never want to deal with USCIS again (outside of petitioning others), I highly recommend getting naturalized, especially with German laws allowing it now.