As a long-time fan of the .300WM, I must say that I don't think it's "too much" for deer...but certainly more than necessary, and definitely not an ideal choice for a beginner shooting a fairly lightweight rifle like a Tikka. I would definitely agree that a smaller cartridge like either of the 6.5mm choices you mention...or any of a host of others like the 243, etc...would be a better idea. Even the .308 would be near the top end of what I would recommend for a beginner.
If you find that your hunting zone prohibits the use of rifles, a shotgun in 20ga might be a better choice than a 12ga, again for reasons of reduced recoil. A rifled shotgun barrel, with iron sights and/or scoped, will allow the use of sabot slugs and should make for at least 100-yard accuracy. You could get a standard pump gun like a Remington 870 which can have interchangeable barrels, allowing you to turn your deer "rifle" into a bird gun for extra versatility. Or, if you prefer a deer-specific gun, a Savage bolt-action 20-gauge gun (model 220, I think?) will handle and shoot like a rifle. It'll kick, too; might look around for some reduced-recoil loads if such are available, even if only for practice.
And, oh yes...practice. Whichever gun type you use, a quality rimfire will allow for vastly increased amounts of practice without huge expense or extended punishing recoil. It's not exactly the same as shooting the "big" gun, but any shooting practice is good, and you can concentrate on steadiness, position shooting, trigger control, breathing...really, every facet of shooting except for the recoil. The .22 is, IMHO, an absolute must.
But, as stated by others, the shooting is just a tiny part of the whole experience, and you will be learning a lot about the rest of it as you go. If an experienced mentor is not in the cards, perhaps another friend or two who are also beginners would make it more enjoyable. You'll have a lot of fun and a lot of laughs, and a lot of mistakes will be made, but it will be worth it. Good luck!