If you go the route you describe this transfer switch will have to be service rated.
230.82 prevents a transfer switch on the supply side of the service disconnect.
That being the case the transfer switch must serve as the service disconnect as well.
Now that the transfer switch is the service disconnect all bonding and grounding must be moved to the transfer switch or before.
The existing service panels must be reconfigured to 'sub-panels' with the neutrals isolated from the grounds.
