  LOFAR (LOw Frequency ARray) has previously detected bursts from the
periodically active, repeating fast radio burst (FRB) source FRB 20180916B down
to unprecedentedly low radio frequencies of 110 MHz. Here we present 11 new
bursts in 223 more hours of continued monitoring of FRB 20180916B in the
110-188 MHz band with LOFAR. We place new constraints on the source's activity
window $w = 4.3^{+0.7}_{-0.2}$ day, and phase centre
$\phi_{\mathrm{c}}^{\mathrm{LOFAR}} = 0.67^{+0.03}_{-0.02}$ in its 16.33-day
activity cycle, strengthening the evidence for its frequency-dependent activity
cycle. Propagation effects like Faraday rotation and scattering are especially
pronounced at low frequencies and constrain properties of FRB 20180916B's local
environment. We track variations in scattering and time-frequency drift rates,
and find no evidence for trends in time or activity phase. Faraday rotation
measure (RM) variations seen between June 2021 and August 2022 show a
fractional change $>$50% with hints of flattening of the gradient of the
previously reported secular trend seen at 600 MHz. The frequency-dependent
window of activity at LOFAR appears stable despite the significant changes in
RM, leading us to deduce that these two effects have different causes.
Depolarization of and within individual bursts towards lower radio frequencies
is quantified using LOFAR's large fractional bandwidth, with some bursts
showing no detectable polarization. However, the degree of depolarization seems
uncorrelated to the scattering timescales, allowing us to evaluate different
depolarization models. We discuss these results in the context of models that
invoke rotation, precession, or binary orbital motion to explain the periodic
activity of FRB 20180916B.
