Going via Åland is definitely practicable--the eastern side is about 80-90 km of shallow sea, which means you need basically a series of trestle bridges similar to the Florida Overseas Railroad (which was 200 km end-to-end). The western side is more difficult, since it's 40km at its narrowest point, which is slightly longer than the Channel Tunnel, and most of the sea appears to be about 200-300m deep. But if you detour around a bit more to the north, you could again use a trestle bridge for most of it, and use a large bridge only to cross the deepest portion of it.

The point is crossing via Åland is firmly in the category of "we've crossed these kinds of gaps before."