Split deliveries & shipping costs

Hi,

We currently offer pre-orders for sale on our site.
We also offer free shipping for orders over $120.

However, if the order is comprised of (for example):
1 x pre-order $90
1 x regular item $40

They will automatically receive free shipping as their order qualifies.
However, we will have to ship the items separately as the pre-order may be weeks away from arriving. This means we pay for two lots of shipping. Currently this is a cost that we absorb, but it is not viable to continue this in the long run.

I would like to know what plugin/upgrade options are available to us, to help solve this issue?

Ideally, in the above scenario, we would like to give the user the two options:
a) Ship items separately and pay for shipping for each delivery; or
b) Wait for the pre-order items to arrive and ship together to receive free shipping

In some cases, one or both of the deliveries may qualify for free shipping anyway, eg:

Delivery 1:
2 x pre-order item ($90 each) $180
Free Shipping

Delivery 2:
1 x regular item $40
Shipping $5

Also, to complicate matters, we have multiple pre-orders that will have different release / arrival dates. So we would want deliveries split based into more than two if possible:

Delivery 1:
1 x Preorder A $90
Shipping $5

Delivery 2:
2 x Preorder B ($75 each) $150
Free shipping

Delivery 3:
2 x Regular item ($40 each) $80
Shipping $5

Would love to know if something like this is possible and which plugins / upgrades we would need to achieve this.

Alternatively, if this is not something that can be done with a plugin, I’m wondering if we can add a couple of product fields (maybe pre-order boolean, arrival date datetime) and restrict what can be added to the basket so that users are forced to purchase pre-order items in separate orders?

Any advice would be greatly appreciated.

Cheers,
Emily