Welcome to TFP!
Using MPS to raise the ORP is not the thing to do since the ORP is supposed to help you figure out the proper sanitation level and MPS does not sanitize as well as chlorine even though it does contribute to ORP because it is an oxidizer. You really should be more focused on the FC/CYA ratio rather than the ORP level.
ORP should be used for process control to maintain a certain FC level but not as an absolute standard.
What is your source of chlorine? Are you using a saltwater chlorine generator (SWG) or a Trichlor inline chlorinator or dosing with chlorinating liquid? Is the pool outdoors exposed to direct sunlight (and how about the spas)?
You say the FC is 3-5 ppm, but what is the Cyanuric Acid (CYA aka stabilizer or conditioner) level? In spite of what you may have been told, ORP is not an absolute level and you get different readings on the same water from different sensor manufacturers. At a pH of 7.5 and 80F temperature, you get roughly a 791 to 812 mV ORP reading from an Oakton sensor and a 803 to 820 mV ORP reading from a Chemtrol sensor.
If your pool has 30 ppm CYA in it, then the 3-5 ppm FC would be around 647 to 672 mV on the Oakton and 689 to 709 mV on the Chemtrol. If you had 80 ppm CYA in the pool, then the 3-5 ppm FC would be around 604 to 626 mV on the Oakton and 655 to 672 mV on the Chemtrol. So your reading below 600 and at around 550 mV seems very strange unless your CYA level is extraordinarily high or the sensor isn't properly calibrated or is just an inferior sensor.
If I were you, I would just manage the FC/CYA level which in a commercial public pool should probably have an FC of around 20% of the CYA level so 6 ppm FC with 30 ppm CYA or 4 ppm FC with 20 ppm CYA. If you don't have any CYA in the water at all, then the chlorine will be quite strong and possibly accounts for your heater damage. You don't want very much CYA in the water, but with none at all then you'll want the FC to be much lower and that can be impractical to manage (such as the German DIN 19643 standard of 0.3 to 0.6 ppm FC with no CYA). You also don't want too high a CYA level because the ORP sensor won't work well at too low an ORP reading -- which already sounds like an issue for your unit as it is reporting quite low (though sounds like it still functions at being able to maintain FC levels).
Richard