CovidLongHaulers.com is a service run by a company called IncellDX, who have developed a test that they say can identify immunologic profiles unique to COVID long haulers.
They've published a study and have created a service that includes testing, suggested medication and in some countries, a doctor referral network for Long Haulers.
Keep in mind that I have no way to confirm or refute that their treatment is effective, other than that I've heard many anecdotal success stories from other long haulers.
IncellDX seems to be "ahead of the pack," at least, for now. They've identified a regimen of suggested prescription medications. These medications can be prescribed off-label; they've been widely available for years and and have well-established safety profiles.
There's a $360 fee to get the blood test, and a $199 consulting fee for each telehealth consultation. You'll also need to have your own doctor who will prescribe the medications for you. The process takes multiple weeks. They don't take medical insurance, though depending on your coverage, you might be able to get some of these fees reimbursed as "out of network" claims.