Redbridge and Millbrook stations are in really bad places to be of much use.
Redbridge village is tiny, and Millbrook station nowhere near Millbrook (Redbridge is far closer). The other problem is both stations are separated from the main residential areas by a six lane dual carriageway (although are linked via subway or bridge but is still a trek) and are very open due to being on the edge of the docks.
For a lot of people, it is much easier to jump on a very frequent bus service and change at Central where the buses drop off practically at the door.
Well that is a very good point with both Redbridge and Millbrook, but the problem of low usage is only exacerbated by a low frequency and poor connections. Say if they were served by services to Bournemouth and Winchester, that is providing a service that cannot be easily done by bus, and might encourage some people to use them, instead of walking to or getting a bus to Southampton Central. Its obviously fair enough not to call faster long distance services there, given that you would add plenty of journey time for suburban stations.
Ultimately, both stations have a reasonable amount of development around them, maybe not the places in their name, but areas nontheless, and providing more attractive services could encourage some people to use them instead of Central station. The reason why Southampton Central is as busy as it is simply because its the only station in Southampton offering reasonably good long distance and short distance services, whereas in other cities like Portsmouth, you have roughly the same number of passengers spread across multiple stations.
However, that is a reasonable point with bus services. Although its deregulated, bluestar and other operators do provide a pretty good service, and SWR generally operate poor local services due to the awkwardness of southampton's rail network, not helped by southampton central being a bottleneck. Unfortunately, you would need a major and extremely expensive rebuild to get any improvement in the stations capacity, and that is never going to happen without any further growth.
However, part of the reason for the low passenger numbers would likely be the fairly infrequent rail service, assuming that the new service also called there more than 15 minutes between it and the existing services then it would make rail significantly more attractive.
That could generate extra passengers from Totton.
200,000 passengers are the sort of levels that a station with 2tph but serving a population of 5,000 could see (OK that is with direct trains to London), or somewhere like Exmouth (population of 35,000) which sees about 900,000 passenger. That's not too say that's a certainty, but it could see rail use double.
One thing which could be possible, would be to have a portion drop off a London service (or extend a train which ends/starts at Southampton) and run to Hythe, which would give direct services to London and so make rail travel more attractive by giving direct services to Winchester, Basingstoke, Woking, etc.
That is a good point, and it is feasible woth the number of paths into Southampton. During peak times, many trains divide at Southampton Central, and then one of the portions calls at all or most stations to Bournemouth or Poole, with the other running fast (non-stop or calling only at Brockenhurst).
Whereas during off peak periods, they divide at Bournemouth, and either a portion is left behind, or runs slow to Poole, allowing the Weymouth portion to skip Branksome and Parkstone. Expanding the practice of dividing trains at Southampton would allow nearly all stations to Bournemouth at least an hourly service to London depending on whether you did it with one or both trains from Waterloo. If the Hythe line was electrified, that could allow one of the portions split at Southampton Central to serve Marchwood and Hythe, allowing direct access to London from two places which currently do not have stations. Of course if such a proposal was to happen, they would need to extensively rework the whole timetable west of Southampton Central, but if it was up to me, I would recommend at least feasibility studies on it.
With that other suggestion of extending a serve currently terminating at Southampton Central, i can see the logic, but it may not be too easy, because terminating trains use platforms 2 and 3, and they can sometimes get blocked, with more than one train often occupying each platforms (e.g. a Portsmouth stopper at one end and a southern service to Brighton at the other), so some repathing would be needed, and potentially more trains required as a result of the available turnaround times, which would likely require additional rolling stock to be used, which isnt too easy to get in a post-covid era in train travel, particularly after the previous government ordered a lot of rolling stock to be scrapped without proper replacement.
Overall, despite the fact there are issues, i do agree that could have been a reasonable solution to having more than just a basic hourly service to Southampton Central. However, one implication of that would be the requirement for the line to be electrified, which would require substantially more investment for a line where the business case ended up being insufficient for further funding and development.