I'm hoping to start a new bedroom scheme soon upcycling some old pieces and your project is an inspiration.

What do you want to use this extra seating for? Is it just for casual chit chat while someone is cooking? Do you need the extra benchtop? Do you want to eat meals here? I think that all makes a difference. You have room for a medium round/oval table in that space which may be more comfortable and flexible. It could be a more casual seating area where people could face the kitchen, I see your dining table at the far end means people have their back to the kitchen. If you really want to extend the bench, I like Feioi and minnie101 ideas re using a different material and more rounded. I would go for wood or colourful laminate and you could have a more rounded/wider end so some people could sit on the end, facing whoever is in the kitchen. A colour would look great, the yellow, orange from your painting would make it zing and look fab! Same if you decide to get a table, make it fun. Beware cold materials like marble or metal as people will be more likely to be leaning on them and those are cold on your arms. I find this even in restaurants with marble/metal tables. Personally I don't like sitting on stools and prefer chairs where your feel can touch the ground, much more comfortable so you could consider making the extended bench a lower level. Remember to post 'after' photos!

I think your existing plans are fine. You could either leave the structure as an interesting oddity or get a carpenter to tidy it up with panelling and a level shelf on top. The sprightly amongst us might manage to perch on top with a few cushions for comfort! Or going back to my thoughts about putting a bed there. I was thinking of a bespoke bed frame that is built over the structure extending beyond the window either side up to the width of a double bed. You could possibly have a deep bedhead incorporating a shelf. A carpenter could advise you on the practicalities of this before you commit.

I agree with @Daisy England regarding drainer grooves are not necessary, in fact the amount of fall is so minimal and so little water drains away you might find having the extra worktop space is more useful, there are worktop drainer boards https://www.victorianplumbing.co.uk/reginox-ceramic-belfast-sink-kitchen-drainer you can get instead which may be more practical. Adding drainer grooves to wooden worktops may invalidate the guarentee so check the small print, you have solid surfaces like mistral, minerva which are solid all the through, but they can/do scratch quite easily (es[ecially the black/dark colours), solid laminates worktops but these usually have a black core so if you add drainer grooves you lose the worktop pattern. If budget can stretch quartz/granite would be a preferable option and just put a tea towel down on the worktop to soak up the water.