Once you have a sustainable process, these last holdout areas like dredging (where you can't use batteries or wires) can have relatively terrible EROI as long as they're a small minority of the economy. All aviation and shipping combined is about 6% of final energy, and with major redesigns for shipping it could be as low as 4%. So you'd trade a much better EROI for 96% of the economy for a terrible one that only affects 4%.

You are looking it from the wrong angle the way I see it.

First there are significant quantities of oils grown used in cooking and discarded, in an often environmentally damaging way.

Reclaiming and burning these adds value to a waste product.

What is the EROI on batteries surely under 1.

These oils can provide energy storage and transmission in applications where electricity is ill suited, jet engines (coconut oil is especially suitable for this) and shipping come to mind.

EROI is also why we really need to get off of fossil fuels long before we actually literally run out (i.e. now would be great)

As a resource reaches depletion, there are two forces acting on EROI: 1) the increased cost of each unit of energy input, and 2) the reduced yield from that energy input.

The EROI curve bends negative faster than you think by looking at history.