Looks nice... impressive work!
You might find that dumping pool/spa water into your septic system will cause some problems... your septic tank needs living bacteria to work properly.. the chemicals in the spa water are meant to kill those bacteria.
Regarding your heater problems, there are dip-switches to set the modes of the heater, do you know these are set correctly? The heater needs to know the type of pump (single vs. two speed) as well as the configuration for blowers, second pumps, etc. Looking at the heater specs, your pump specs, and your piping sizes, it's not likely that you really have a low-flow situation here.
From the heater manual troubleshooting section:
dr, dy and/or HL or LF error indications. This could be due to incorrect A5 & A9 settings. Check that you have software revision 43
or higher (third number to flash on start up). Use previous table on page to determine A5 and A9 settings, if software revision is 43 or
higher. If less than 43, then you need different instructions. Contact us.
If this is not the problem, then these are typical indications when there is insufficient flow going through the heater. Check that
proper voltage has been provided to the pump receptacle (i.e., that the W1 wire is set to RED AC if pump 1 is a 230 volt pump). If
this is not the problem, check that are no air locks or flow blockages. If you have a small circulation pump (one with 1” Unions for
example (such as a Laing or Waterway Tiny Might), it may not be delivering the prerequisite 23/12.6 USGPM (5.5kW element requires
23 USGPM 3 kW element requires 12.6 USGPM). The flow either needs to be increased or the wattage of the heater element
needs to be decreased. If you have a 2-speed jet pump, in addition to the circulation pump, the spa pack can be re positioned to the
jets circuit. Contact us.