Synthetic Full & Wasabi In most of the cases simply specifying full backup for the second plan in chain should help, but make sure you have a a pretty big Windows between the runs to make sure the software is able to finish one full backup and start the next one. Basically, the second plan only needs to have full backup scheduled and that's it.
Now, regarding the rest of your questions:
Synthetic full will appear just like a regular full, so you don't need to worry about that. As for 6 months of block-level backups, this actually might be a problem. A small one, but a problem nonetheless.
The thing is, purging only happens upon full backups and only in chains of full+block, and if you're setting up a time-based retention purge of the oldest chain will happen only when its last block is old enough(let's say 90 days), meaning that previous backups will be much older and they'll still will occupy your storage space.
My suggestions on setting up your retention policy:
Since synthetic full will take much less time to complete I recommend running full backup once a week and use your existing retention settings. That should be optimal in terms of purging, versioning, used space and the amount of data kept on storage side without triggering early deletion fees from Wasabi.
Regarding full backup size: It'll be the size of a proper full backup, but the software will reference previous full + blocks to merge all previously uploaded data with changes that have been performed since the last plan run. Synthetic full is usually about 5 times faster than regular full because of that. On some configurations I got even better results.