That's a fair question but it doesn't really save you any money, and just costs more for a breaker to switch back and forth. Everything still needs to be inspected and permitted to the same level of rigor because it is still technically wired to the grid and you just have more elements which could malfunction in a less standard configuration

If a hardwired interconnect to the grid is what triggers your permit/license requirements, then forget that I threw out the "backup" functionality. Ignore the grid connection and just treat the monthly service charge as another tax.

Or if you can get away with having a power inlet on your off grid system and powering that system from the grid with an extension cord during temporary backup/maintenance conditions only, go for that.

For my own AHJ, grid interconnect has nothing to do with whether permits/licenses are required (and for homeowner work, they're not). So an off-grid system and grid-connected are under the same requirements there regardless. A grid-tie system that can feed back power to the grid requires additional signoff/agreement with the POCO though.

(I'm very sympathetic to the situations where the utility will demand some stiff fee to build out a grid connection to a new residence, while local regulations require this connection for occupancy. But this isn't what you described)