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I'm not sure what specific implementation of deduplication you're referring to here. There are a variety of contexts related to Reflect where deduplication could potentially be useful.
Dropping your retention policy will cause you to lose some older backups because you'll only be retaining for example the latest 3 backups rather than the latest 5 (or whatever), but that's the nature of reducing your retention policy. A Synthetic Full however CAN grow larger than it needs to be because although the Synthetic Full will grow as needed to accommodate new data, it will not shrink if for example you merge a backup into it that included the deletion of a large quantity of source data. Instead, data blocks of the file that are no longer needed for storing "real" data will be flagged as scratch space and can be used to store future "real" data. But if there's never enough such data to use that space, then your Synthetic Full file will simply be larger than it needs to be. Macrium does not currently offer a mechanism to "compact" a Synthetic Full. So unfortunately the only way to get back to a smaller Full is to create a new one. If the space on your NAS doesn't allow you to do that before purging your existing Full, and therefore all of its child backups, then I would suggest either copying those backups elsewhere beforehand or perhaps capturing the new Full to some other location first and only deleting the NAS backups after that backup succeeds, at which point you can copy it to the NAS.
Scenarios where you don't have a way to store more than one Full backup are not ideal. Even Incrementals Forever with Synthetic Fulls may occasionally need to create another Full, e.g. if the source disk's partition layout has changed and it is therefore no longer possible to append to the existing backups. You probably don't want that unexpected need for a new Full to require you to delete all existing backups before that new backup can complete. But the only real answer to that is to increase destination storage.