@Kenneth Hill good question — for UAV tracking specifically, running all four simultaneously is generally worth it because the geometry improvement from having 30+ satellites in view at once tends to outweigh the noise concerns, and modern multi-constellation receivers handle the signal separation pretty cleanly at the hardware level since each constellation operates on distinct frequency bands.
That said, in practice a lot of UAV builders settle on GPS + Galileo as their baseline — Galileo's signal quality is excellent and it plays nicely with GPS without the inter-system interference headaches you can occasionally get when GLONASS is in the mix, since GLONASS uses FDMA while the others use CDMA and some receivers handle that combination less gracefully under high dynamics. BeiDou is worth adding if you're operating in Asia-Pacific where coverage density is high, but in North America or Europe the marginal gain is smaller.
With the GNSS module's 18Hz fix rate, you really want that satellite count high to keep the position solution stable at speed — so if your firmware and power budget allow it, I'd honestly start with all four enabled, log some raw data, then compare against GPS+Galileo only and see if you're actually gaining anything meaningful in your specific flight area. The processing overhead question is mostly a non-issue on the module's dedicated GNSS chip since it's doing that work in silicon, not on your main application processor.
