Sounds like inflammation and blood clots are the problem. Could an anti inflammatory like ibuprofen significantly reduce risk then?
"Among the more common lingering problems — affecting up to one-third of all patients with long COVID — is postural orthostatic tachycardia syndrome (POTS), which causes the heart to beat abnormally fast. [...] in people with POTS, the blood vessels may not constrict appropriately. Instead, the heart rate must rise even more than normal to compensate for the “lazy” blood vessels, says Lee Lewis. Conversely, the parasympathetic nervous system kicks in when the body rests, instructing blood vessels to dilate and instructing the heart to beat more slowly so the blood pressure goes down."
"Similar to POTS, if patients remain sedentary for too long, Anagnostopoulos says, they could develop cardiac atrophy; decreased stroke volume, meaning that the heart doesn’t pump enough blood out of the left ventricle during contraction; or compensatory tachycardia, characterized by resting heart rates that exceed 100 beats per minute."
A blood thinner or anti-inflammatory isn't going to treat the POTS or sedentary effects, and deprivation of oxygen is going to do longer-term damage to the heart. If you did use an anti-inflammatory, you'd have to use it at a low dose for a long period of time (months), but that's still not addressing the other issues.
Specifically vitamin D deficiency makes things worse and most people are at least mildly lacking.