I personally avoid dynamic disks since for all of the additional features and flexibility they offer, they also impose restrictions and create other hurdles. I've also never needed any of the features they offer, so my "keep it simple" mentality has kept me from them. When I need more storage, I just buy a larger disk rather than spanning onto a new one, the latter of which means your file system is now dependent on the proper operation of more than one disk. But Macrium has a KB article about dynamic disks here
that you may find interesting.
I don't know of any ransomware that uses BitLocker specifically. Some ransomware encrypts individual files (rather like EFS) instead of the entire volume, which would of course still work on a dynamic disk, but even the ransomware that performs full volume encryption wants to retain a way to explain why your drive is now encrypted and where to send payment to regain access to your data, which BitLocker would not provide. So I would not consider a dynamic disk an effective ransomware deterrent. But you're capturing Reflect backups and storing destination disks offline or at least running Macrium Image Guardian, aren't you?
You may also want to look into the newer Windows Storage Spaces technology, since I believe that's meant to supplant at least some of the use cases for dynamic disks, or possibly even all of them. I'm not sure since again I tend to avoid these technologies since I have little use for them and generally try to keep things as simple as my use case allows. Storage Spaces has its own set of issues though, including availability in WinPE. I remember seeing threads about Storage Spaces volumes created in Win10 1709 not being visible in Rescue Media. That could
be because Rescue still uses WinPE 10 1607 as of this writing and that release doesn't support Storage Spaces (or a sufficiently new version of it to see 1709-era Spaces), in which case that limitation will presumably disappear eventually -- or it could be some other issue that may not be resolved so quickly.