I'm worried that if I submit a new sitemap for my newdomain, it will
get crawled without getting the link juice from the olddomain
Site position goes down, because Google may index your new site content and does not remove the old one in a same time, it take some time to understand that 301 redirection is done to this new URL, so we should transfer link juicy to this new URL, so in beginning all webmaster always suffer from lower position, but it will back soon once Google understand your both URL properly. Google also have change of address tool, which can speed up the process very quickly, because 301 redirection is done on page level, while this tool is dedicated at domain level.
Should I keep a sitemap with the olddomain links?
Sitemap have nothing to do with ranking/linkjuicy. 1) you can redirect your old sitemap location to new sitemap location. 2) you can keep the old sitemap as it is, and Google will follow 301 redirection automatically 3) you can also add the newdomin link to your older sitemap. I have not done any case study to find which is clear winner, but with proper 301 redirection, Googlebot will surely remove the old one and index the new one automatically.