The now-canon description of the Force structures it into two components: The Living Force, which is fed by the life energies of all living beings, and the Cosmic Force, which is fed by the Living Force and surrounds and binds all things that exist, be it tangible or intangible, physical or spiritual.
This suggests that the strength of the Force does indeed depend on the overall strength of all life that feeds energy to it and keep it in existence. So in theory, the power of a Force user's abilities does depend on the strength of the Force itself. However, the sheer amount of life in the galaxy feeding the Force suggests that any significant fluctuation in the power of the Force would be beyond your average Force-user's ability to experience. Probably, even if some galactic plague or famine etc. strikes and halves the amount of life in the galaxy, most Jedi and Sith aren't so outrageously strong to the point that they would experience a "performance hit". Not even Anakin or Sidious. Unless you are powerful enough to, say, blow up a star with your mind, perhaps.
The description by Darth Bane is becoming obsolete as Canon and Legends become separated. The "venom cup" analogy is valid under the version of the Force where the strength of the practitioners of the light and dark side are dependent upon the concentration of their respective side's power in each practitioner - too many Jedi dilutes the light, while minimal Sith concentrates darkness beyond imagining. This analogy I've only heard a couple times years ago - these days Darth Bane appears to establish the Rule of Two in both Canon and Legends for different reasons, but somewhat similar reasoning. While Legends Bane established the rule in opposition to the ways of the Brotherhood of Darkness, Canon Bane established the rule to address the previous Sith Order's flaws of greed and in-fighting. Both versions seeked to address the same issues, though: Minimise/remove the Sith Order's presence and visibility in the galactic public eye, and ensure that each successive master is more powerful than the last. Modern reasoning for the Rule of Two have nothing directly to do with the nature of the Force, or the source of its power.