The problem Southern have (as do various other London area commute operators) is that demand has generally returned Tuesday, Wednesday, Thursday. (But not Monday & Friday).
So depending on choice of statistician, at macro level, looks like around 30% of demand has vanished, and DfT seem to treat this as can make do with less rolling stock.
There are two separate questions, one is should Monday & Friday be treated more like Saturday for timetables and tickets (a debate not for this thread), and the second is how do you provide the extra trains for the busy 3 midweek days.
You don't need to be a genius to work out that if a train only does one up rush hour service, sits in a carriage siding near London during the day, then does a return, and on Thursday evening is parked for 4 days, it is not getting heavy utilisation. The Southern Railway recognised this decades ago and used to keep some older stock for full length peak hour extras between normal services.
So realistically Southern needs some older EMUs for peak hour extras, (because never going to justify cost of renting newer trains for low utilisation) when train is needed for 6-10 hours a week, rather than nearer 60-100 hours a week.
So obvious choice is some 25-30 year old EMUs spare from other lines, except you will struggle to find any built 1993-1998. Bit too late for networkers, bit too early for new generation trains. If go older is it economic to overhaul some stored networkers, if go bit younger get 357, 375, 458
The networkers never really worked on Southern, but maybe some could be revitalised (although there is risk lots of expensive work would be needed, and delays to 166s refurbs suggests extra work is likely). Some 357s might be released by C2C if they take on some surplus Anglia 720s, but possibly can't be used on 3rd rail. 375s (oldest is now 23 years old) don't work as not spare, 458s might do (although have had troubled existence)
Anything newer eg 379s displacing 378s is probably not cost effective unless lease rates fall