My guess is that the lighting fixtures are blocking most of the light and you increased light source intensity to enormous extent to battle this and that leads to extreme overbright pixels that are nearly impossible to get rid of. Try to temporarily hide lighting fixtures, normalize all light sources and see if the problem with fireflies goes away.
This is the experiment I made, I disabled all the lights, then turned them on one by one.
- First picture only ies lights + sun (hdri sky + light material disabled) no sign of fireflies on the picture
- Second picture ies lights + light material + sun (hdri sky disabled) no sign of fireflies
- Third image hdri sky + sun only (ies + light material disabled) I can see some white spots
-Fourth picture hdri sky intensity raised to 25 + sun at normal intensity (ies + light material disabled) I can see some fireflies
It looks like the hdri sky is the culprit.
Any solution?