Sorry, I don't know what a peristaltic pump is. But, most baitwell pumps
I have seen contain a check valve that would not let the water siphon out whereas a bilge, when turned off or not under water, just becomes a hose and with a head of water above, if its not pumping its going to be siphoning.
In addition most bait pumps create self priming suction (like with a rubber ob water pump impeller) that will actually draw water up to the impeller from a dry state. Many of them can be used for both a bait pump and a washdown pump with either a "Y" or second output. A bilge on the other hand will just centrifically throw the water up a tube until its turned off (at which point the water in the discharge will just fall back in).
500 gph is normally enough flow for that size livewell, but I would go to either 700 or 800gph on your setup because of the height of the top of the tank that you need to push water to. It takes a stronger pump to push the water to a greater hieght so maybe the 700 will actually give you 500 flow. My tank on the floor (pictured above) is probably 20" lower at full.
I don't see why you can't mount one externally like you've done, so long as you're not worried about the possibility of snapping it off and losing it on kelp or something. Tender baits like anchovies and sardines are particularly suseptible to changing temperatures and oxygen levels.