Unarmed Defense doesn't help.
Martial Arts still has some effect, but not as much because you already have natural weapons.
Unarmored Movement only works if you're not using a shield.
Deflect Missiles and Evasion still use Dex
Unarmored defense does become redundant, but unarmored defense is there to make up for the fact that monks have been disallowed from wearing armor. A Tortle Monk has an additional option for having a solid AC other than pumping both Dex and Wis. That is all.
Sure, the Tortle already has a d4 damage die on unarmed strikes, but being a monk will still gradually upgrade them and let you make lots of bonus action attacks with them so that there is actually a point to using them. Being a tortle means you have the option to do bludgeoning or slashing damage with your unarmed strikes, unlike a normal monk who can only bludgeon.
Deflecting Missles with low dex works almost as well as with high dex, because you also add your level and a d10. You won't be the worlds greatest deflector of missiles but you will still deflect them better than any non-monk. Actually dumping Dex is probably a mistake, but a Tortle Monk with a mediocre Dex and high strength will do just fine. At level 5 a Monk with 20 Dex will deflect missiles for an average of 15.5 damage, whereas one with a Dex of 12 will only deflect 11.5 on average, which is still usually going to be the full damage of a conventional ranged weapon. You will get to completely negate the damage at throw it back less often, but that is rarely worth the ki at this point. By higher level where you might actually have the ki to spare for throwing it back the difference becomes increasingly negligible, as adding the Monks level increasingly overshadows the dex modifier.
If you have low dex you need the half damage on failure part of Evasion all the more. And it is an ability based on saving throws for which you have proficiency. Your proficiency mod is already +3 by time you get Evasion and only a couple levels from being +4. Let's say that at level 10 a character gets hit with the firebreath of an adult red dragon. It is a DC 21 dex save against 18d6 damage. A 20 Dex Monk will save for zero damage on a natural 12 and otherwise take half for 31 damage average, for an overall average of 17 damage. A 12 Dex Monk will save on a natural 16, meaning an overall average of 23.5 damage after Evasion. Meanwhile a Dex 12 character of that level in a class without Evasion will generally take an average of 55 damage if they are proficient in Dex saves, and 61.4 damage if they are not, the whole panoply of other abilities that might be relevant notwithstanding.
