Targeting humans with bioweapons may have been genuinely difficult but will probably become much easier for several reasons like the following:
Indoor airborne aerosols are now known to be a significant (and likely dominant) vector of transmission in fast-spreading pandemics
Bioterrorist groups like Aum didn’t try hard enough (like using different types and variants of dangerous viruses and letting evolution do the rest)
LLMs, agents, and other digital tools might make some amount of rational design and digital evolution of viruses easier before release
During a high-mortality (from 10% to 100% lethality) epidemic or pandemic, most people might not trust some of the pathogen-agnostic defenses that you mentioned such as far-UVC lamps, because it would be extremely difficult to verify that they were correctly installed (e.g., adequate amount of lamps per room), maintained (e.g., far-UVC emitter still works at adequate efficiency), and run (e.g., adequate air mixing which would depend on fans or HVAC systems). After decades of use, this trust issue might be resolved, but unfortunately, we may not have the luxury of time on our side.
The only things I’d trust today and for the near to medium-term future would be elastomeric respirators and powered air-purifying respirators (PAPRs) which can be verified to work by individuals (using fit tests) and orgs (like NIOSH) that test and certify respirators. And unlike massive infrastructure investments that will take decades to adequately implement if they ever get off the ground to begin with, stockpiling respirators would be cheaper, faster, and just as or more effective.
A better scenario would be that the problem of high-mortality pandemics will be solved automatically via complete automation which will enable the complete but relatively-comfortable physical isolation of every person as needed. This is now more likely to happen in the near future due to the rise of tech like LLMs and humanoid robots.