Don't listen to anyone who tells you to wait or do extract batches first. Yes, the all-grain brewing process is more complicated than extract. But only
slightly. And if you know what those complications are, you can overcome them with minimal effort.
It's perfect you have experienced friends/mentors who can take you through a short "apprenticeship" before you start to fly solo. I always recommend this approach.
There is no real difference in calculating carbonation between bottle-conditioning and force-carbonation. You're still trying to achieve a certain level of CO2 in the beer; you're just using two different techniques to get it. Force-carbonation is definitely more rapid; you'll be able to drink the beer more quickly. But as many veteran brewers will attest, that's not always a good thing! There's such a thing as 'green beer', beer that's done fermenting and fully carbonated but not yet at its peak of flavor. There's also the fact that certain styles which are traditionally bottle-conditioned, and in fact are significantly improved by laying down in the bottle.
So I encourage you to learn both techniques. Start like you want, with kegged, force-carbonated beer. Brew beers which don't require a lot of aging to lose the 'green' character, like Mild, Bitter, Brown Ale, Dry Irish Stout, that sort of thing. Save the Pliny clones for when you've got patience to age them.
Yes. Take a gravity five minutes before the end of the boil and check against the projected OG your software tells you to expect. If it's waaaaaay off, you can add some dry extract to bring the gravity into line. For this a refractometer is indispensable. I don't know what I'd do without mine. Every all-grain brewer should use one.
Buy DME in one-lb bags. Easier to add in increments. When I started AG brewing, someone gave me the advice of buying some DME "just in case", and I did. I just made the mistake of buying 3-lb bags because it was less expensive overall. I ended up throwing away expensive DME because I didn't use the whole bag.
If you
do need to add some DME to overcome a low OG, ad the whole thing. Don't listen to your calculations when it tells you to add 12 ounces. Add the whole 16. Yes, you'll be slightly over your OG, but that's better than waaaay under, and you won't end up throwing away extract.
Or you could save it for starters. Completely forgot about that!
(By the way, it's "Erlenmeyer flask".

)
Cheers,
Bob
P.S. Welcome and
GOOD LUCK!