Those customised ones from the OEM builders of filters are probably ok. There are H13-H14 grade, so definitely over 99% one lah (99.5% is H11) even if you allow for some degradation. They make filters for ISO clean rooms, even the lousiest and failed audit grade also more satki than the typical DIY HEPA webby that you can find like 5000 sellers.
If you DIY with an even thicker/more depth filter, try ULPA U15 for an approx. 20% price premium. Good for those more powerful grade air coolers (> 400m3/hr specced so that you can buffer a bit down) and stuff like your Cornell fans.

No need 2.5k SGD IQAir liao esp if you don't dig the gas control. (actually as shared earlier, you can DIY those also, other than the activated charcoal)