On the topic of happiness:
I had Golbat evolve at Level 23 around Fiery Path every time without anything apart from Soothe Bell.
Caught Zubat on Level 12, had it in against Brawly and May on Rt 110 but not against Wattson. I backtracked once to Granite Cave but didn't explore much else, no grinding involved but also no fainting.
Tbh I was very surprised when this first happened and I think that happiness was changed somehow, it´s just very unusual to happen that smoothly. Step count should be not far away from 1000 even with the bikes, level gain of 11 is quite low as well...
Now to sum it up: It might be very possible to reach required happiness with your starter when you equip Soothe Bell(no favouritism, you get one so you can use it). But I don´t think this will influence Treecko very much, its much more important for Torchic and Mudkip to get strong elemental STAB early, at least before the fight on the volcano.
Also I used Torchic, with soft resetting for good nature and stats(20/12/10/13/9/10, naughty), but it had a modest early game and doesn't get excellent for a long time. It just lacks bulk and speed to survive against Wattson, the team boss, Flannery or Norman(no Blaziken yet or you are severely overleveled). Then you get Megachick which is cool but has a difficult time against flying, psychic and water gym, contributes everywhere but not in an excellent way. It only excels in the Elite Four, but I would´t call this struggling performance S tier. Hoenn has never been a great game for Fire or Fighting types and chick is both... Mine was high B tier.
Edit: I would always fish for a lv. 15 Magikarp which needs around 27 encounters to get with 75% certainty, those five extra minutes are worth it if you don't want to grind. Or you can always skip the advantage against Flannery and Norman and catch it on Lv 25.
I even had fun with Bulldoze Gyarados(needed some grinding to get to lv 20 from lv 18 so I fought all trainer around ) against the Mauville gym.
Wattsons Sturdy Magnemite Twaved my cherry G. so I swept him, because Voltorb does not OHKO and Magneton has Magnet Pull.
This is the very definition of S tier, taking down even the biggest enemies without fainting, being able to switch in against every neutral effective enemy and taking low damage while dealing high damage against every foe. Only the delay of Dragon Dance for Crunch was a downside.