I'm not sure those changes free up many 700s. Luton - Rainham via Catford looks to take the same time as via Greenwich based on the Gillingham - Victoria (to Bromley South) and Sevenoaks - Blackfriars (from Bromley South). Turning back at Beckenham Junction saves 1 unit at most two and Maidstone via Bromley South takes a bit longer though might not need an extra unit. To introduce the originally planned Maidstone would need at least 5 units so expect there still be a shortfall.
How workable it is in general would depend on passenger numbers and what changes Southeastern make in future. An all stops Rainham - Luton via Catford and Sole Street could be overcrowded as with redirection to Blackfriars it's unlikely to be overtaken since the peak Maidstone East services currently go via Catford. Pre-pandemic the Rainham services had some merit, the Cannon Street lines had capacity constrains further in so the flat moves didn't really impact overall capacity. These days there's fewer peak services to Cannon Street so diversions are easier. Also likely to be quite a big timetable change, as Blackfriars Junction services aren't all paired by service group so rearranging via London Bridge to via Elephant isn't trivial.
I'd personally expect services to be more like May 2022 longer term, with the Orpington - Luton only operating at peak/school times, no Thameslink Littlehampton services or extensions to Maidstone with maybe East Grinstead services added back. The May 2022 timetable needs 100ish class 700s in service according to my code. Potentially the Rainham - Luton services could turnback further south at Kentish Town or Cannon Street off peak but unclear how much this would actually save.
I'd hope May 22 is not the long term plan, Leaving a whole load of stations in Zone 2/3 with a 2tph service into london is pretty poor. I understood the lack of Orpington to Kentish Town service at the moment is down to a shortage of drivers.