Is my visa application plan for Thailand sensible following my wife’s operation and our travel itinerary?

One word of warning. You are meant to remain in the location you apply from until the visa process is complete. The Royal Thai Embassy, London reserves the right to request additional documents / interview in person at the Royal Thai Embassy, London, and amend visa categories as deemed necessary as well as reject any application without prejudice.

So you're here now on 60 day visa exempt entries because your current extensions expired when you were out of the country

You will leave Dec 2 and come back to thailand Dec 7th also on 60 day visa exempt entries,

then you will leave, go to vietnam on the 26-30th of Dec and hopefully come back on the 90 day Non-O visa you got online from the thai consulate in the UK (that you applied for when you were there on the 3rd)

Does that about sum it up?

Totally sounds like a workable plan, looks like none of the 60 day visa exempt entries you're coming in on will max out (which is what flags immigrations to look at your entry/stay history)

AND

I would imagine you already have the travel to Vietnam booked so if you were asked when you come back in you could show onward travel (not that I believe you will be asked)

Should work, just depends on if they approve that Non-O visa fast or not from the thai consulate in the UK when you apply, <- because IF they do you have to use it to enter the country when you come back on the 8th (and you could still travel as planned just have to buy a re-entry permit to get out/back without losing the 90 day stamp)