Your pic is not available.
Did you recently fill?
Remove filter.
Do have good flow from the return?
That's quite a declaration. Please explain how you determined this.
I am usure about the flickering, but the rest could ba a number of things.
The flickering sounds like a shorting button in the topside, but if so would have nothing to do with the heater. So it maybe a fluctuating temp reading consistent with a sensor or heater issue. Or a power (main or low voltage) issue from a failing breaker or transformer. If you unplug the topside that system will run automated functions. Unplug it and see if it heats. If so, it's the topside.
Everything else says air lock, faulty sensor, dirty filter, or power issue.
Post some pics of the circuit board, wiring diagram, and equipment area.
Loosen union nut on heater to bleed out any air.
Check topside wire for damage from mice (very common).
Test incoming power at the spa with a digital multimeter. Verify voltage is steady when heater is on and topside is flickering (bad breaker).
Turn on spa light and see if it also starts to flicker (bad transformer).
Remove heater and check for obstructions/debris.
Unplug sensors and test with ohm meter set in 20k range. It is very hard to get the tester probes on the wires in the connector, but it can be done on the side of the connector. Post results.
Always turn off power before reaching into that box to touch anything.