All the Thames/Kennet valley stations west of Reading could easily change at Reading for the turn up and go frequency fast GWR services into Paddington. And they lack any direct service to the airport, even RailAir/ National Express only stop in Reading Town Centre/Mereoak Park and Ride at M4 J11 and they take an absolute year to get to Heathrow. Not to mention the volumes commuting to Slough and Maidenhead from those station, both of which have significant town centre business district and Slough Trading Estate, a regional draw, is reasonably close to Burnham.
The outer suburban trains from Paddington which now run only to Didcot also service the ‘cross-Reading’ demand and they used to give the stations west of Tilehurst along the Thames Valley a direct, no-change, link to Oxford.
You seem to assume that the only places people living west of Reading wish to travel to are Reading itself, Heathrow and London. While these are important they are not the only flows. The Oxford Corridor traffic studies emphasise the increasing importance of Oxford; the cross-Reading flows, such as Goring to Maidenhead or Slough, are also significant - not everybody gets out at the Reading stop.
You also write off the Railair coach from Reading station to Heathrow as being slow. It is just as slow, due to traffic density and fixed and variable speed limits, or as fast as all the other traffic on the M4 — at least as far as Terminal 5 . Pre-Covid the Railair coach ran at 20 to 30 minute intervals depending on the time of day and even with the reduced level of air services operating from Heathrow at the moment it is still running an hourly service. It is now scheduled to take 50 minutes to Terminal 5 and just over an hour to the Central Area. This is about 10 minutes longer than previously due to a couple of additional calling points compared to days of yore and the interminable road works on the motorway.
The Railair coach is competitive in time with a car or taxi from the central area of Reading, but less so starting from further out. But then a train from central Reading even if it halved the time of the coach journey wouldn’t be able to compete on overall journey time either.
The other constraint will be the capacity of the four Relief Line platforms at Reading. Reversing another four trains per hour as is suggested for the Heathrow service will start to be difficult since during the peaks Crossrail will also be reversing four trains per hour. In addition space needs to be kept for the two per hour outer-suburban Paddington - Reading - Didcot (and Oxford when the wires get there) trains; as a bolt hole for the Cross-Country trains if operation of the Main Lines platforms gets congested; space for the Basingstoke trains to reach the maintenance depot during the day (which they do); and the not insignificant number of freight trains. Add to this your suggestion that the local Reading - Didcot - (Oxford) service should also terminate at Reading would seem to be a hostage to fortune even if the two train per hour outer suburban service is abandoned.
Projecting any potential Heathrow trains through to Oxford is also a bad idea as Oxford, as it stands, is an even worse place to reverse trains than Reading.