I can't speak for the OP, but as someone with immune issues (and other issues, which complicate things) I can say for myself (many of these apply just as much at home as when traveling):
- we travel at less busy times so crowds are less and try to avoid "crowds" as much as possible (I don't like especially crowded places anyway, so this is good

A WDW "crowd level" of 1 or 2 is my style; 3 or 4 is the max I am comfy with; 6+ and I really prefer being back at the resort; there is no way I could handle a 9 or 10)
- I try to keep a buffer distance between me and other people. e.g in a line I might leave a couple feet between me and the person in front and my sister stands behind me and have some space between her and me to create some more space; now that I use an
ECV at the parks, this also helps create "space"
- I consciously do not touch things like handrails, door handles, poles on buses, etc unless I have to do so for my safety or because it is mechanically/rules of physics required (e.g. opening a pull door; push doors OTOH can often be opened with other body parts, like feet or clothing-covered elbows; for buses we have waited until the next bus so we can get seats, but I have issues mobility-wise standing on them anyway)
- I wear long sleeves at the parks, no matter the weather, and don't lean on tables as much as possible (this takes conscious effort)
- at restaurants, due to food allergies, I wipe down the table in front of me anyway
- I carry hand wipes and sanitizer and use them frequently along with regular hand washing
- I don't shake hands with people (still working on what to say to explain that one, as at work it can be a challenge)
- while I now have to wear it anyway for other reasons when outside, at WDW I wear a filter mask when outside and much of the time inside
OP: Two trips ago, I had a great conversation with a guest services CM at DHS as I was getting my
DAS card. He spend probably 15-20 minutes talking with me and a good chunk of that conversation was tips and ideas for avoiding crowds/crowded spaces at the parks. He had some excellent ideas and some suggestion for where e.g. to watch parades or fireworks that were often less crowded than other places.
I can't speak to avoiding specific parts of rides, as I don't like and thus don't go on either HM or ToT, but for example for the muppets show you are in a big group outside the theatre waiting to get in; I stay off to one side and towards the back away from the crowd as much as I can; having an ECV helps with this anyway; but there are CMs in the room you could talk to.
For the MK parades, I found a great place to watch them from: near Liberty tavern there are some benches along the building. When I didn't have the ECV, I just sat down on those benches. Yes, you are not up close, but up close = the masses. The benches means people are walking in front of you as that becomes the only open pathway, but if you choose well you can find a bench that is more out of the way than others (assuming others are not occupying the space). The benches are elevated a bit because they are on a porch-like area, so you actually get a pretty good view. With the ECV there was an area on that side of the street and one on the other side where I could be back some from the curb but because of natural physical barriers like shrubbery and boulders and such create a little buffer from most of the people; one of the sites a lot of kids were quietly sitting on the ground in front of the
scooter (probably 5 or 6 rows thick of kids) so no view-blocking issues. The trade-off of course is you are back a bit and if someone decides to stand up in front of you your view may be blocked. I also found an area on main street where one of the little "side streets" is that worked really well for parade watching: i could park on the "side street" and sit happily there quietly and undisturbed and watch the parade; since nobody was near me watching it, I could get out of the ECV and stand up if my view got blocked (which it did occasionally by people walking on the main street sidewalk which was between me and the viewing crows and main street itself). Again, compromises, but I have no need to be either up front or any desire to be in that crowd.
And we have no issue staying put in those spots (bench or side street or beside that boulder) to wait until the crowds die down -- it can be a nice break to catch up with family, do some planning to the next day, check email, whatever. e.g. I stayed on the side street while my sister went into some of the nearby shops. I was content to occupy my time people-watching and on my smartphone. When she was done the crowds had thinned considerably.
We also like doing things like watching MK fireworks from one of the MK resorts -- much less people. Plus it lets us watch the electric water parade.

GF has several places to watch from (I like the dock area, but sometimes the flying bugs can be a bit much), we've enjoyed from the WL beach and FW beach, and Poly has several places as well. It is a different view and may be partly obstructed by things like trees, but still quite nice !
We make use of Fastpasses and short lines as much as we can, as well as using DAS to get return times. FP lines are often helpful because they minimize the amount of time you are in confined spaces with large numbers of people. For example, there was one ride I remember where the standby line zigs and sags back and forth through basically a huge long room and when there is a long SB line you spend quite a while in there; the FP line doesn't do that, for the post part it was pretty direct to the ride with very little zig zag. On another ride, the FP did have zig zag *but nobody in it* so while you walked it it was a continuous walk not stand and wait and shuffle.
We take advantage of extra magic hours as we find the parks are less crowded during them. Late at night can sometimes be good too, once you get past bedtimes for little ones. A parkhopper can be helpful for taking full advantage of that: one trip I only ever ended up going to Epcot in the dark
For a similar reason we found the Halloween and Christmas parties (on less full nights) to be good: we ride the rides and many have little to no wait; we also pick and choose the other activites we want to do. On our last trip the only park we did was
MVMCP on one day: we arrived soon after 4 and stayed until the park closed. For us those nearly 8 hours was worth it; for others it may not be.
For meals, we tend to eat at "off" times not during the peak times (I bring some safe snacks anyway which come in handy). it makes for a more pleasant dining experience all around.
SW