First are you unchained? If not, well you should be. Talk to your GM. In my opinion the unchained rogue works well without archetypes, YMMV.
Instead of taking Knife Master and multiclassing just focus on dependable ways you can use Sneak Attack. The Scout archetype gives you a reliable way to apply your sneak attack damage. Additionally I would take the Underhanded Trick talent and then Greater Dirty Trick at level 6; use dirty trick to apply blindness and then sneak attack away.
The Minor and Major rogue talents can be particularly useful. Minor magic presents a number of good options, Detect Magic could offer some nice utility, Prestidigitation offers a little utility and something for RP, and Acid Splash can be used for acid damage sneak attacks. Vanish is typically the best option to take for Major Magic; it's duration is short but it could still prove useful for scouting and setting up sneak attacks.
The Positioning Attack talent can be useful for getting yourself into a flanking position; it also works well with the Scout which at 8th level gets a sneak attack should they move more than 10ft before attacking.
If you're going to play a small race it might be worth considering taking the Fast Stealth talent particularly if you want to be scouting ahead of the party. The Gloves of Reconnaissance could be useful for scouting though they've been erratad since I played using them. Getting a raven or thrush familiar via Familiar Bond could also be useful for scouting since they can fly and talk.
Unless your GM is particularly fond of flanking you won’t miss the situational benefit of Uncanny Dodge. I’lost count of how many hours I’ve played of this game and in all that time I’ve only been flanked once or twice. There’s a good chance that you’ll find yourself moving and attacking far more frequently than benefiting from Uncanny Dodge so it’s worth the trade for the Scout archetype.