While trading back with the Jacksonville Jaguars would net the most value and keep the Chargers in a safe range to take either Justin Herbert or Jordan Love, a trade with the Arizona Cardinals would be a safer option if the Cardinals fall in love with a prospect.
This trade would likely be for Derrick Brown, Isaiah Simmons, or Jeffrey Okudah if any of these players fall this late in the draft. By jumping the Carolina Panthers, the Cardinals could get their desired elite prospect at a position of need while giving up minimal draft capital.
The Chargers, on the other hand, would gain a decent upgrade on a later draft pick without giving up on selecting their quarterback of choice.
Of course, this minor trade back would only be completed if the Chargers are committed to selecting a quarterback rather than a blue-chip prospect (such as Simmons or Okudah).
With nothing but an upgrade of a late-round draft pick to gain, this trade back might not be worth the additional risk as opposed to simply selecting a quarterback at six.