The best air filters I have found are the IQAir HyperHEPA filters, which are rated to filter ≥ 99% at 0.003 microns, which is smaller than a virus particle. That's why these are commonly used in hospitals.
See: https://www.iqair.com/us/air-purifiers
I went down a rabbit hole when California nearly burned down in 2020 and after lots of research came to the conclusion that buying 7x $100 hepa filters is better than buying 1x $700 filter. So an inexpensive Winix ended up being the best for me. One for each room. More air cycles and much less noise when they're scattered throughout the house. This is just for particulate matter. If one cares about volatile organic compounds, VOC's, they need to invest is several pounds heavy charcoal filters. The thin ones that come on most filters don't do anything other than act as a pre-filter to catch the large particles.
Yeah they are expensive but the best I have found. The 45i is for like 700 square feet for $400. The nice part about Alen purifiers is the filters last just about a year and lifetime warranty
I'm curious about what supplier and system you're using? I'm currently using a bunch of smartair purifier, that gets indoor pm2.5 to close to zero even when it gets really bad outside every year (also in Thailand). But considering installing a positive pressure (ducted) system with HEPA filter due to high co2 levels (tightly sealed house and no ventilation)
I am using Vornado air purifiers, but I don't have air quality monitoring yet and don't care about WiFi features. The higher end Vornados have monitoring built-in, but it is rudimentary and just a light bar. Without air monitors to check them, it's hard to tell, but they seem to work quite well. Vornado excels at producing room-sized torrents that direct air through its filters. We blew out a candle once and the smoke visibly made a beeline towards the Vornado.
https://alen.com/products/alen-breathesmart-75i-air-purifier...