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We present a comprehensive, two-decade, multiwavelength variability study of the blazar PKS 1510 - 089, one of the most prominent and extensively monitored flat-spectrum radio quasars. Using Fermi-LAT γ-ray data together with Swift-XRT and UVOT observations spanning 2005─2024, we trace the long-term evolution of its flux, interband correlations, and spectral behaviour across the optical, X-ray, and γ-ray bands. We find that all flux distributions prefer a double-log-normal fit. In case of the optical bands, this may be due to contributions from the accretion disk. The range of fluxes in a given band, as well as the fractional variability values are in-line with the expectations that high-energy parts of a given spectral component are more variable than low-energy parts. No obvious cross-correlations exist between the bands over the 20 years of observations. The X-ray and γ-ray spectra are variable, but do not show any trend with flux. These results are suggestive of different zones being active in the jet of PKS 1510 - 089 at any given time. In a previous paper, we used the same techniques to study the high-frequency-peaked BL Lac object PKS 2155 - 304. Both sources follow the aforementioned trend on the energy-dependent variability of the spectral components, as well as the lack of significant cross-correlations between the studied bands. While PKS 2155 - 304 exhibits a harder-when-brighter behaviour in its high-energy part of the synchrotron component, no such behaviour could be found in PKS 1510 - 089. Both sources show orphan flares, which can seemingly happen in any band. In summary, the long-term studies of these two sources reveal that the underlying physics is similar in these apparently different source classes, even though variability patterns keep changing and remain unpredictable.