Make sure you find out how big of a fork your frame can handle.
The changes in geometry could help a bit with downhillin. It’ll raise the front up a bit and bottom bracket. A higher bottom bracket means that you’ll clear rocks easier, but won’t turn as good. You can get used to that. You’ll just learn how to turn better instead of relying on the geometry of the bike to help you out.
The fork will be weird at first on the uphills, but you can get used to that too. I have an XL GT IDRIVE 5.0 that has a marzocchi 66 fork on the front of it. It has about 7" of travel. It took a bit of getting used to climbing, but all you really have to do is lean in more towards the handlebars and use your legs and hips more instead of pulling on the handlebars. It’s not as hard as people say it will be. And as far as standing up and pedaling on a mountain bike in concerned…….i don’t think many people do it unless they are racing XC. Getting a fork with adjustable travel and lockouts will just end up costing a lot more money for something that you probably wouldn’t end up using enough to warrant the price difference. I’d just get the fork you want and get used to it. It’s not that hard to change body positioning while riding uphill.