My 2c here.. been there often because i don't keep my cars beyond 3-4 years, i get bored and i also don't want them to lose maintenance plan. I have 0 intention to buy parts for a car...
Firstly, you never get extras back. Always view extras as a "gift" to yourself, with no refund. Car A with R10k extras and exactly the same Car B with R100k extras are gonna be offered very similar trade-in values. My last Volvo had R228 000 of extra and they ignored them like they didn't exist on the trade-in... i expected that.
If you don't want to keep a car long, try a lease or GFV option. I like it because i don't want to keep my car and i don't see it as an investment i just want to pay to drive it, and one day it must go away.
That said, obviously some cars keep their values better. BMW's not too great - In 2015, I tried to trade-in a 2014 BMW 320i with 22 000km on the clock. It was spotless. Not a mark on the body.
2 BMW dealers offered me R300k below the new price and 1 dealership in the Northern JHB suburbs said it looks like the car had been rolled and re-built, and offered me R500k below new price lol hahaaha. I mean i never even bumped a door on it. Anyway, moral of the story is you will always lose because that's how they make their money. The lower they can get you, the more they make.
If you are looking for trade-in assist, some cars get deeper discounts because they don't push large volumes like Audi A5s, Mercedes C-classes, a few Chinese brands also offering a lot off like Tank 300 etc. Sometimes also they won't say they're giving trade-in assist, but will do so if you need it.
Ford does a lot of "interest-rate" discounts. So for example Car A costs R 200 000. With finance, they would make R100k on interest. They discount the interest and make R50k on the installment, and offer you the 50k as a discount, or trade-in assist etc. Just be careful though cause some of those interest rates are temporarily low and they climb...also prob won't allow a lease for that, only outright purchase.