@aeverett530 What stands out to me (and you even mentioned it) is the possibility of Lyme disease Lyme can affect the heart and nerves and cause seizures. It just depends on where it goes in the body as to which symptoms it can cause and they call it the great imitator because the symptoms mimic other diseases sending a patient down a different path. The main Lyme bacteria is a spirocheate that is spiral shaped and drills through tissues. There are a lot of co infections with it that are other bacteria or parasites. It is controversial among heath care providers as to how to cure it, but most agree that antibiotics are needed asap after a tick bite to prevent chronic infection in the first day or so. A person should also use antibiotic ointment on a tick bite immediately because that can be absorbed through the skin and call your doctor. The doctors who have made it their specialty to treat Lyme patients are in the ILADS group and they will treat it aggressively with antibiotics over months or couple years because it may take that long to treat all the co infections. Lyme can hide behind biofilms in the body untouched by antibiotics to emerge later continuing the infection, so timing of treatment is important. Unfortunately, many Lyme tests are inaccurate, and infected patients go undiagnosed, but there are specific labs that specialize in Lyme testing.

The other thing that stands out is thyroid function. Have they evaluated that completely? An over active thyroid will speed up the heart and cause anxiety type reactions in the body. If there is any mass on the thyroid that could affect it's function that would be easy to see with imaging.

Here is information that I have saved with links about Lyme that may be of help. You might not be getting enough information about it from doctors outside of ILADS. Did you have a tick or other insect bite before your illness began? If you watch the documentary "Under Our Skin" you'll see patient stories and the symptoms they have had of extreme disability from Lyme and how they recovered. Patients can also have a bite they didn't notice any many do not get the typical bull's eye rash from Lyme.

Patient story from the executive producer of “Under our Skin”
https://www.vox.com/the-highlight/2019/6/18/18677511/lyme-disease-diagnosis-health
https://www.lymedisease.org/lyme-basics/co-infections/babesia/
Here are some links that describe the challenges with testing for Lyme disease which isn't just one specific organism causing an infection, but many. The accuracy of a blood test is also affected by how long it has been since exposure to infection, and if the body has mounted an immune response against it, and how that would change over time. The Igenex test covers more bands in comparison which you can read about from the link below. The best answer to your question would be from an ILADS doctor who could explain the limitations of testing. For some of the co-infections that accompany the Lyme bacteria, there are no diagnostic tests and the doctor evaluates the patient physically in addition to testing they do. They might test to see how many specific white blood cells are responding to an infection because Lyme can destroy the white cells that are trying to protect the body by fighting it, and the test would show a reduction in the white cells.
https://www.ilads.org/research-literature/controversies-challenges/
https://iladef.org/education/lyme-disease-faq/
https://igenex.com/the-igenex-advantage/
Here is some information that might be helpful about Lyme Disease.

The Western Blot test is not very accurate and does miss diagnosing Lyme Disease. That is a costly mistake to make if Lyme Disease goes un-diagnosed for years. The doctors who are the experts at treating Lyme are in the ILADS group. The Igenex lab has more accurate tests. https://igenex.com/ Lyme destroys white blood cells that are your immune system.

Lab in Germany https://www.arminlabs.com/en

Here is a link to ILADS (International Lyme Disease Association). https://www.ilads.org/patient-care/ilads-treatment-guidelines/

Here is the Lyme treatment guideline
https://lymediseaseassociation.org/wp-content/uploads/2009/08/BurrGuide200810.pdf
There is an educational documentary call "Under Our Skin" which profiles several patients and their treatment and controversy about Lyme. https://underourskin.com/

Here is a link that discusses ILADS antibiotic recommendations and for recommended number of days, scroll until you find it.
https://www.tandfonline.com/doi/full/10.1586/14787210.2014.940900
Exercise helps because it raises the effective oxygen perfusion rates in your tissues, and oxygen can be toxic to bacterial infections. Getting your Vitamin D levels optimized helps because that increases your immune system response.