Fair point - I completely forgot to take the 3D geometry into account. I guess this could be solved by either making both sp³ (sub the Si-O with Si-Cl) or both sp² (sub the H-O-Si with H-N=Si)? But then writing data becomes more complicated than just adding or removing hydrogens that, as you said, isn’t as simple as it looks like.

There are already several synthetic DNA base pairs that can be used instead of the naturally present bases.

Like the dNaM / dTPT3 pair, right? That’s perhaps more viable, at least to increase information density.