With 20+ sites I would always put an additional MX-Pair as a concentrator into the DC and use AutoVPN. It will make everything so much easier.
Otherwise, I would go with the "one peer" approach. Although using wildcard PSK is not a best practice, the main problem with them is the logistics problem when the PSK is compromized. But this is not the case with the cloud based config where the PSK can be easily changed for many devices simultaneously.
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