Welcome @gothman. There are different subtypes of MDS, some having more of a tendency to potentially progress into a more aggressive type of leukemia than others. So treatment will depend on your particular form of MDS.

MDS, or Myelodyplastic Syndromes, is actually a group of blood disorders caused by blood cells that aren’t formed properly or are not functioning. Here are 3 of my go-to articles that do a good job of explaining the condition:

From Mayo Clinic:
https://www.mayoclinic.org/diseases-conditions/myelodysplastic-syndrome/symptoms-causes/syc-20366977
~Healthline:
https://www.healthline.com/health/cancer/mds-hematology#symptoms
~Verywellhealth:
https://www.verywellhealth.com/what-are-myelodysplastic-syndromes-mds-2252548#:~:text=Myelodysplastic%20syndromes%20%28MDS%29%20are%20a%20group%20of%20bone,how%20the%20bone%20marrow%20creates%20healthy%20blood%20cells
You’ve already been through one leukemia journey and most likely not relishing the thought of another. Hopefully your condition stays stabile and won’t require treatment.

You asked about a bone marrow transplant cutoff age. I personally know several people who have had successful BMTs at 75. It will be dependent on overall physical and mental health, and if there are any underlying comorbidities.
The preconditioning chemo and the riggers of the transplantation itself can be challenging to the body/organs.
If you do precede to a BMT, having a larger teaching hospital, where the bench is deep with medical professionals who have extensive SCT/BMT experience works in your best interest.
As for finding an MDS doctor, often their specialties as listed in bios. Again, a larger teaching institute might give you more options for a specialist. Otherwise, a trusted hematologist oncologist should have the background to treat MDS?
Are you able to use the same hematologist oncologist you had for your HCL? Has your doctor talked about a potential BMT in the future?