I stated at the start of my post that there are still a number of issues with Simple Delivery.  Everything else in my post is correct so I don't see why it is ridiculous.

'It is impossible to know the size and
weight of the package until

1) The buyer has completed all purchases'

Combined postage whilst possible with Simple Delivery is still one of the main issues with it.

'2) The buyer has expressed their wishes, in terms of packaging and
delivery dates - SD labels expire!'

How the item is packaged should be down to the seller.

Estimated delivery dates are determined by eBay and there is very little leeway whether you are using Simple Delivery or not.

All postage labels expire if not used within a certain amount of time.

'3) The buyer has had the opportunity to switch to Collect once they
realise you are nearby'

If you wish to offer collection then that option needs to be added to your listing and the buyer needs to select it at the point of purchase otherwise you won't be covered in the event of an INR claim.  Nothing to do with Simple Delivery.

'4) The buyer has the opportunity to request expedited delivery, which SD
does not offer (Tracked 24 does not offer a delivery guarantee)'

Yes, Simple Delivery doesn’t currently offer a guaranteed next day delivery option but then most buyers wouldn't require that when buying secondhand/collectible items from a Private Seller.  It does, however, offer an 'Express Tracked Delivery' option (Royal Mail Tracked 24) which has a 1 day delivery aim including Sundays if they want the item quicker.

'5) An overseas buyer asks for a parcel to go to to UK address to save
costs - very common occurence'

If an overseas buyer wants the parcel to be sent to a UK address then they need to provide that at the point of purchase otherwise you won't be covered against an INR claim.  Again, nothing to do with Simple Delivery.