Having been in a similar situation ("I need to resign or work part time"), I think the best thing you can do is show awareness of the effect it will have on your coworkers and suggest ways in which this will work out.
For example: "I suggest I keep the 60% of my job that is the most onerous for someone else to take and/or requires the most prior experience and/or is best matched to my skills. As for the other 40%, I suggest task A should go to Fred - he is ready for more experience, it will be good for him. Task B can be phased out when the new project is completed anyway. Jane has agreed to roll in Task C with the similar Task D that she already performs".
I have been doing this for several years, and it has worked out great for everybody. The only issue is whether the tasks you give up are important for your career development - sometimes they are, so you will have to make some serious choices.
For example: "I suggest I keep the 60% of my job that is the most onerous for someone else to take and/or requires the most prior experience and/or is best matched to my skills. As for the other 40%, I suggest task A should go to Fred - he is ready for more experience, it will be good for him. Task B can be phased out when the new project is completed anyway. Jane has agreed to roll in Task C with the similar Task D that she already performs".
I have been doing this for several years, and it has worked out great for everybody. The only issue is whether the tasks you give up are important for your career development - sometimes they are, so you will have to make some serious choices.