I had a DX6 G2, upgraded to an ix12 after I won one in a club raffle (when that happens, and all of the rest of your stuff is Spektrum already, why WOULDN'T you use it???).
My dad has a DX8 G2, and I sold him my DX6 G2 when I got the ix12, so he could use it for buddy boxing (he's just solo'd for our club and is ready for flying on his own now). We use that because there's so many people at our club that use it and know about it; the only people I know out here in the area that fly FrSky are drone guys, and even then, there are a fair amount that fly Spektrum or use DJI radios (in fact, a LOT of people who fly drones that I've run into are flying DJIs).
Futaba seems to be favored amongst the 3D pilots and the Heli pilots at my field, but honestly? They're all kinda "sticks to stir when you're flying" transmitters in a lot of respects. It gets down to features, ease of use, comfort and feel/ergonomics - this last one is a HUGE HUGE HUGE thing for me; a mini drone that I bought before buying my racing drone had this cheesy little DSMX remote that FELT cheap and tiny in my hands. It's a good thing the batteries only lasted a few minutes, because flying it for anything more than about 3 minutes and my hands felt like they were cramping on the controller. If you plan on flying a bunch of different things with the same radio, make sure it feels good in your hands or else you're going to hate it and wish you'd spent a little more for that comfort.
No, and it only works if you have an RX that includes sensors with it. Battery voltage is useful but a timer works just as well and the Apprentice has built in low voltage warning, like all the SAFE type planes.
If you do go DSMX then Lemon RX make superb receivers, including a full telemetry one, which have excellent performance and are cheaper than the Spektrum ones. They bind great as well. The smallest 6ch one is 9g, I have them in all my FT planes. It’s under $15.
Unless you are flying larger aircraft with flaps, retracts and differential thrust you won’t need an 8ch. The DX8e is pretty pricey for a “beginner” transmitter. I would be careful that your costs don’t get out of control. There were numerous options for half the price of that TX in this thread. If you have never flown before I would get some flights in on the club trainer and try a few TX’s before dropping so much money.
I would also want to be sure flying was for me before dropping €200+ on a plane and transmitter. If you have an instructor to buddy box you, the Tiny Trainer is a great learner plane, easy to get and can be built for half the price of the apprentice. SAFE will teach you bad habits if you rely on it, whereas an experienced instructor and a predictable 4 channel with no gyro will make you a better pilot vs learning on your own with SAFE.