Is the speed sensor necessary or not?

For the small extra cost, get the speed/cadence sensor bundle. It’s not an ‘issue’ with the device, rather a limitation or GPS around buildings and trees. If you want accurate speed, use a sensor. I say get the bundle because the speed and cadence sensors are pretty good. They are accelerometer-based and don’t rely on magnets. 

Yes.  Get the sensor.  You'll get more accurate distance and speed, and if you use auto-pause, it will work much more reliably.  If you mountain bike over tortuous paths and in dense woods, it's even more important.  We have 6 bikes and all are fitting with speed and cadence sensors that have been acquired over the years starting the GSC10 magnet based speed and cadence sensors to the more recent magnetless sensors.  They work equivalently well, though setting up the magnet based sensors is a lot more finicky.

Ok. I’ll take the speedsensor. No need for the cadance sensor and I still have 2 heart rate belds.

Is the price difference between the 530 and 830 worth it?

That really depends on how much you will use the navigation.  The 830 can search and route to POI, where the 530 can not. All other capabilities are the same besides how to interact with the device, touchscreen vs. buttons.

But can handle the Shimano Di2?

Is the touchscreen not more user friendly as the buttons?

Personally I like the touchscreen better for things like searching and interacting with the map.  Which is why the POI are not available on the 530.  Since I upgraded from the 810 and do use the POI, the 830 was worth the additional price.  Navigating the menu on the 530 is very similar to the FR and Fenix watches which is pretty easy to use until you try to enter long strings of characters.

Aeroad_Raf

I had the 520, and wanted to upgrade to either 530 or 830 for better navigation. I chose the 830, and I'm glad I did since it's so much easier tapping the screen to navigate through Garmin's endless menu levels, than it is pushing buttons.

The cadence sensor helps your Garmin account work out how much effort your putting in. If you’re free wheeling down hill it doesn’t know without one of these. It can use HR data but cadence improves your analysis of your workout. 

Speed sensor is essential for GPS dropouts. 

830 is better than the 530. I had the 530 for a year (given to my partner now). I loved it and didn’t think I needed the 830. I treated myself to an 830 last week in a sale when the price was good. It’s waaay better than the 530 in terms of the user experience and navigation. I wish I’d made the switch sooner. Well worth the extra in my mind. 

Does a speed sensor improve navigation? ie remove the navigation lag if you're in dense trees