I am new to CPAP and have no experience of surgery; I'm sure others with more knowledge will contribute but I have a couple of points for you. My understanding of recurrent bacterial and fungal infections elsewhere in the body is that an optimally healthy immune system (including the skin and mucous membranes) is able to protect itself from pathogens with a variety of strategies. You have no doubt heard that bacteria live on our skin and in our gut all the time. I don't know if the sinus cavities are expected to be sterile all the time like, say, the peritoneal cavity is. It may be that after many infections, the mucous membranes are thinner than they should be, and are not as efficient a barrier as they should be. Unfortunately it's not as simple as removing the pathogens and preventing the inhalation of new ones. You need physiology that functions well. An example of this is acidophilus in the gut - the right number of 'good' bacteria acts to balance against the 'bad' bacteria. When we take certain broad spectrum antibiotics, we can wipe out both the bad and the good bacteria, so that when the populations recover, there are too many bad bacteria. So, we often have to take acidophilus capsules or yoghurt to restore the numbers of good bacteria in the gut.
To my mind, you are doing everything right already, in terms of changing filters and so on. But you may want to ask your doctor for any advice he can give on strengthening your immunity. Seeing an immunologist may help, as many doctors have not had much training in this. I think that many doctors would just say have a healthy balanced diet, minimise stress, get exercise and get good sleep. Well, you are already doing all that I imagine. Carefully selected mineral and vitamin supplementation may help - most doctors would probably be dubious about this point, but given your history, an immunologist may prescribe specific things to assist you. If you have deficiencies in key minerals or vitamins (can be found in blood tests) then it makes sense to reduce eash of those risks.
The biggest positive thing you can do for your immune system is to have your sleep apnea under control and you are doing that. You may need another sleep study if has been a while since your last, just to ensure there are no other sleep disorders reducing the quality of your sleep, and that your sleep apnea is still well controlled. This is absolutely critical for your immune system.
When you say these last few years have been rather hectic, do you mean that you've had numerous infections, or that you've been working long hours, dealing with major life changes etc? The reason I ask is because the other major immune system support is having a balanced, emotionally fulfilling life where stress is managed in a healthy way. Most of us have some barriers to this, such as heavy work or family responsibilities, anxiety issues, or bereavement. Strange as it may seem, emotional or mental health can be linked to recurrent or chronic infections. We can't change the fact that we will have (for example) beareavements in life, but our bodies sometimes force us to look at things - I know that has happened to me at times.
One last thing. You mention doing rinses when you have finished doing work outside. Is this because of dust, dirt, pollens, that sort of thing? You may also want to wear a protective mask, one that is graded appropriately to stop particles coming in from outside.
"Attitude is a little word that makes a big difference" - Winston S Churchill
Machine: Respironics M Series Pro, 17cm
Mask 1: Respironics Comfortgel nasal
Mask 2: Resmed Quattro full face
37yo female, New Zealand, Mixed SA, AHI 45