I'll preface this by saying I'm not a builder, or an architect, so whether this is cobblers I don't know, but in my head it makes sense.
I think the South Stand is alright unless we are solid low mid table premier league or better for a couple of years, and if we did do something to it, for me it'd have to be building backwards up, new insides, and a new front (that'd be great) because it'll be extending depth wise into the car park. Take row TT and add on. Another 10 rows is roughly 2000 extra seats based on my quick check of the ticketing site, seats are 1-193, so 15 new rows would be 3000, and 20, 4000, you can scale it, and the height change wouldn't be massively severe, probably a bit taller than the hotel (there may be some windows blocked though).
Stick in some new corporate stuff under the roof line perhaps.
I think that way it could possibly be almost done in a summer with minimal impact to a season, the roof would need to be rebuilt which would be I imagine the most pressing thing, and then you could block access to some rows at the back for game days while work continues during the week if need be. I think Man U did a stand this way, the stand itself remains largely untouched, stuff is added and joined.
The Kop though is a state, that needs doing ASAP, again, work from the back, so start building the new back and concourse areas in the little carpark about halfway through the season, and then final day start to demolish what you need to of the exiting Kop and start joining them up in the close season.
I've got it all worked out in my head haha, there'll be a mid height corner for the Kop and South Stand, and above it there'll be an office building thing to keep McCabe happy, and the Pukka Pies corner will be rebuilt to join neatly with something, with new TV facilities so you see the South Stand on TV, and corporate shite.
More disabled access is important too, it'd be nice if we designed that in, if people want to come, it'd be nice to be able to have them.
A good version of this hasty sketch