So I think there is no point in adding achievements unless they actively encourage and motivate the players to play the game. And your idea about killing Raditz 100 times is pointing in the right direction. One of the highlights of this game are the TMQs, but they are simply not worth doing a lot right now and most of the time in general sadly. Especially in lv60 cap almost all of the best gear comes from the CC dungeon.
So why not give some real incentive to spam them by giving out significant rewards?
Lets say you have to complete some amount of the highest level TMQ in the present cap and if you do that you get some type of gear, take the class weapons for example. The best ones right now are from CC because they are lv60 legendary and have the highest base stats. You can only get them from doing CC100 multiple times. And the second best is the lv58 legedary weapon you can drop in TMQ but it is worse because you get less stats and less effects because they are not lv60. So what if you make an alternative version of the TMQ weapons, reskin them (which should be possible since you add new dogis right? maybe just recolor them into something cooler looking, make them all gold and shiny or something) and readjust their stats to match those of the CC ones. Maybe even give them some kind of desired stat for the class but dont max it out.
This would also alleviate the pain of free to play players who always complain about not being able to catch up, and more importantly allow classes that cant do CC100 runs easily to obtain the best available weapon. CC100 already rewards you with rare dogi balls and you can still get the weapon there if you chose to do so. But I think an alternative (a cooler looking one at that) would be very welcome.
You can apply this concept to any kind of gear but the class weapons are the easiest to do because they dont give any additional effects when farmed in the CC dungeon.
Also if you want to encourage doing other TMQs or dungeons you can make it so you can only start working towards the weapon after you did some of the other stuff that rewards you with all kinds of titles that people have come up with.