So, I want to know, does Sirfetch'd always crit when using a crit-boosted move, holding a Leek, and also with Max Friendship? Also, the game I'm playing is Pokémon Sword.
Yep. Weirdly enough, Bulbapedia answers this question exactly in the Trivia section of this Leek page. It doesn’t even need max friendship if you have the Leek and move, or it doesn’t need the move if you have max friendship and the Leek.
From Generation VI onward, Farfetch'd, Galarian Farfetch'd, and Sirfetch'd will always get a critical hit when using moves with a boosted critical hit ratio while holding the Leek.
Also:
If a Pokémon has maximum affection (Gen VI-USUM) or friendship (PE, Gen VIII onward except LA), the probability from above is multiplied by 2.
Hope I helped!
Bulbapedia's Page on Critical Hits
Tl;dr yes.
In Pokemon Sword, critical hits are guaranteed once a Pokemon reaches a +3 critical hit modifier. Leek on Sirfetch'd yields a +2 to critical hits. A maxed-out Friendship value doubles the chance of whatever stage the critical hit ratio was previously on. At +2, the crit chance is 50%, so max Friendship would double that to 100%.
For a complete list of modifiers to the critical hit ratio, check the Bulbapedia article.