There are various parking options depending on what is suitable to individuals.
Queens Road Car Park is a flat £2.50 on a Sunday. Only ever seen it full in December. 1.5 mile walk with bus or train (to Haydons Road) options.
Off road parking in multiple roads at about 0.5 miles away. Either no restrictions or Mon to Fri until 6:30pm. Industrial or park so not really used by residents. Parking available during week days, I suspects lots more on a Sunday.
Earlsfield station is closer than Wimbledon station, around mile (with bus options). Tooting Broadway Northern Line about a 20 min walk. Haydons Road 10 min walk but closed about every fourth Sunday. Kent travellers I suggest Northern Line on those days.
For the supporter from Egham you could walk, I did this year! Seriously I’d drive M25/A3 or cut through if you prefer. Should be 45 mins max if traffic reasonable.
I’ve driven to lots of places in London in the last 18 months that I’m not familiar with. Top of the list is can I guarantee to park. So I know the concerns of being “stuck”. Wimbledon wouldn’t worry me as there are many options. My only reservation may be is the driver mobile enough to walk a mile. If so he/she can worst case scenario drop off less mobile passengers at the ground before parking.
Based on the projected attendances I honestly think the local non residential roads will more than suffice. Unless the attendances are huge and even then I assume a lot of away support with very limited numbers of them driving I’d guess.
There actually is a lot of parking in the ground. The catch is that it’s available if only the main stand (4,000+) is used. If away fans get their own stand, it’s tricky, but MIGHT be feasible. Lots of cars in the stadium (officials, players) etc for AFC Wimbledon ladies with attendance around 1,600. No segregation though and beer allowed (not allowed at men's league games of course) at our seats