I have been experiencing this for >2 years and have found that pacing myself is the only way to prevent/lessen the degree of crashes. PT,OT,SLP was the biggest help in identifying why I can feel so great, charge forward in my life, then am in excruciating pain with overwhelming & incapacitating exhaustion for 2 days after.

I started with a simple spiral/composition notebook & wrote down everything I did in a day on one page:

Sleep: could I fall asleep, stay asleep, up frequently? Do I feel rested on waking? Did I get hit with exhaustion at 10am and have to go back to bed. Do I need to nap during the day? When did I start feeling tired during the day. What time I went to bed.
Cognitive/Brain work/Thinking: making plans for the week, managing personal finances, work emails, grocery list, planning-social events, travel, train schedules, driving directions, legal work, packing & travel, appointments, reading, puzzles
Physical work: grocery shop, errands, gardening, gym, walking, laundry, meal prep/cooking/clean up, cleaning up the home, wash/dry/set my hair, PT, walking the dog.
'Crash' episodes: what time of day, how long, what it felt like

Next to each activity I mark whether it is Cognitive or Physical, and time spent doing that activity.
On the facing page of the notebook, identify specific symptoms you had during the activity: breathing was harder, coughing, had to stop the activity, fast heart rate, dizzy, off balance, exhaustion, muscle pain, headache, nausea, difficulty concentrating, memory, vision changes, depression or anxiety.
I could then see where my energy was being used. I am now at a limit of 2 hours of physical work and 2-3 hours of cognitive in a day. I know I will crash when I go beyond this.
I do simple meal prep/cooking in one day along with laundry but no thinking work so Im not cooking every day. I count that as a workout. I shower, do my hair, pick out clothing & accessories the day before an outing and do nothing the day of so I have energy to enjoy. My house is not immaculate, just picked up. I get groceries delivered. Only 2 appointments in a day. I use a timer on my phone to help pacing my activity.
My long-haul symptoms have not improved-my time management and pacing has, and that has made all the difference in the world. Of course I still have crashes, but the more I stay in my own limits, the less frequent and less severe they are.