It's a common huge problem with people that want to ctb, worrying about what will happen to parents afterwards.. It is often a problem with out any quick and easy solutions..
I have to deal with it to some degree too with my mom, but I probably don't have quite as good of a relationship with her that you do, but she still relies on me tons all the time constantly, and she has said before she would probably end her life if any thing happened to me.. I'm going to use h2s method which only requires mixing 2 ingredients together, and part of me wondered if I should leave some extra bottles of it for her and instructions on how to do it in my ctb note.. But still not really sure..
And yeah in these situations it always seems like a battle between love for your parents/family versus your level of suffering.. On one hand you don't want to deeply hurt any one, especially not people that care a lot about you, but then on the other hand you realize there is no way you can actually continue struggling through all of your suffering you are going through..
In a way I envy those whom have a bad relationship with their parents because it makes it that much easier to ctb with out regrets..
I also envy people with siblings because I feel parents aren't as likely to be
as depressed or suicidal if they have other children they can comiserate with..
As far as real practical solutions....I'm not really sure.. I will list a few things I am thinking, but none of them are that good though..
1: You could just wait longer, as much as you can, until your suffering gets so unbearable that you emotionally start not even caring about whom you hurt..
2: You could try to communicate more with your mom to try to get her to really understand what you are going through and telling her that you feel like there is no possible way you could live your whole life of like 80 years being like you are, and that nothing you have tried ever helps.. Could ask her things like "What would you do if I accidentally got hit by a truck tomorrow and I wasn't here any more?".. And maybe when you ctb she will know that you really were suffering and couldn't stand living any longer..
3: Could try to do a method that would seem like an accident.. Although I'm not sure what that would be.. Although I wouldn't recommend the "just disappear" thing where you go to some remote uninhabited place to ctb, because that just causes the parents agony from months/years of searching for you and never knowing if you are still alive some where or not or what happened to you..
4: I'm not really serious about this one but you could try to turn in to an annoying piece of crap so that your mom would start not even liking you that much any more.. Or just slowly distance your self from her some how to where you barely even communicate with her some how..
5: You know when some thing devastating has happened in your life before, like some lover person that you've been with for years breaks up with you and you cry for days about it?.. It always seems like to me after I get done crying I feel better.. So I'm thinking, yes your mom is going to tremendously suffer, but for how long?.. She would either suffer greatly at first but then try to keep living despite it, or she would end up ctb her self.. If she ctb then her suffering wouldn't need to be that long, she would just need to find a proper method, and you could include that in any sort of ctb note I guess.. And if she decides to continue living, then you can conclude that at least your death didn't hurt her bad enough that she felt like she couldn't continue living, and also, time heals all pain (at least a one-time pain like that).. Every day that she goes through she will probably heal from it like 0.01%, and after the first week of your death she probably wouldn't be crying as much as she was at first and would cry about it less and less as time progresses..
6: This is an extreme option but you could just state to her honestly that you are planning to ctb and give her your ctb dates and every thing, and tell her that when it comes down to it you can't live for other people and ask her what she plans to do after your death.. This is kind of an alternate version of idea #2.. You have to be pretty strong in order to actually do this option, because if you aren't she is going to try every thing possible to convince you to either change your mind or push back dates over and over etc, and if you aren't strong you will just end up telling her the lie of "okay fine i'll try to keep living".. This option does have the advantage that, if she really can't live with out you, she would probably say so at this point, and it could lead to her telling you that she wants to die with you together, and while that would be a bit odd in some ways (parent and child ctb together), it would also allow you to not have to worry about your mom after your ctb..
Welp, that is all I can think of for now.. It isn't an easy problem to solve.. I hope that some of what I have said can help you think of what might be best for your dilemma..
Love and light