Fruit trees for a shady spot in frost pocket

I think the unhappiness of all of the varied mix of existing fruit trees is telling you just how bad the conditions are for them.
Too wet - suffocated or rotted roots, probably the soil is full of root-rotting fungi which will make it difficult to establish new trees in the same soil.
Not enough sun - no energy to grow, heal their rotted/diseased roots, nor fruit.

You'd be better to improve the growing conditions first, such as raising the soil level by a foot or two (or at least giving the trees a large mound or ridge to grow on), so that the trees can grow roots out of the saturated soil, which at least will restore something resembling normal root function.
Also to give the fruit trees more light by trimming the big trees which are blocking sunlight.

When all else fails, go for maximum vigour in an attempt to outgrow the problems.
In the UK (I have no idea about other countries) I'd suggest something like the MM111 (maybe M25 if soil improved/raised so no risk of waterlogging), Pyrus, Myrobalan, Brompton rootstocks and with vigorous varieties in the hope that they'll be strong enough to overcome problems.

But planting more trees where the existing ones are without raising them out of the horrible soil is unlikely to be successful as the existing ones are struggling and the saturated soil around them will be heaving with root-rotting fungi which will make short work of a small baby tree as it tried to establish.

I would be tempted to investigate which traditional/commercial varieties are grown locally (i.e. Pays de la Loire?). There must be some French fruit enthusiasts out there in the aether.

Can you put your new trees somewhere else?