Didcot-Oxford is the easy one, and there are actually some block closures of the line in 2025 related to Botley underbridge rebuild, during which could put up portal spans in Oxford area (even if wires are many months later). From memory a new track layout at Oxford is being installed in stages 2025-2026
West of England combined Authority (WECA) wants to do Filton bank to Temple Meads, but with current rolling stock, as others have pointed out is rather pointless. Work is currently taking place on Bristol Temple Meads roof until 2026, due to scaffolding not practical to put wires up until then.
South of Temple Meads to Parson Street area, there is still track layout and signalling alterations needed, but ultimately there are groups who would like to see electrification extended to Portbury/Portishead, Weston super Mare or even to the huge new factory at Gravity (near Puriton, south of Highbridge). But again without EMUs or BEMUs or electric freight locos, nothing could use it.
WECA is looking at another possible reopened station, St Anne's on line towards Bath. Filling in Chippenham (actually about mile east of Chippenham) to Bristol allows the bi-modes IETs to run as electric. A reopened station at Corsham is also possible. So is probably more usable short term than wiring Filton bank.
Realistically to utilise the section through Bath properly, need to have BEMUs on local trains too (and line through Bath typically sees 4-6 trains each way per hour). There have been suggestions that wiring to Warminster and Frome is logical as could use spare EMUs on local services.
There is a bit of muddle about platform lengths associated with Bristol area electrification, WECA seems to be opting for 150m (to allow pairs of 3car units) on newer work, many stations are currently nearer 125m (which allows 5 x 23m, but means need a messy mix of 2car and 3car units), and some want 165-170m (and I think the proposed new South Wales services which might hourly extended to Bristol) will be built to this (giving option of pairs of 4car EMUs).
From memory project Churchward was not getting 2car units, just 3car local units, and a longer regional unit (5 or 6car) with common equipment. To me being able to operate pairs of standard EMUs on the fully wired sections justifies 170m platforms, even if 150m on unwired extensions where BEMUs will operate is normal length.
One final thing to note there are already some masts up near Chippenham, and a number of mast foundations already installed in the area. I think all the clearance work (bar one) has been completed between Chippenham and Bristol, and only a handful of structures from Bathampton to Warminster that need either raising or track lowered.
. I agree with a lot of what you say too!