Hi there, I have a 22 month old and a 6 month old and I feel your anxiety. What I usually do is schedule the time that baby is due to come out of the sleep cycle (ie in your case @ 30 min) and play with the toddler outside away from the bedroom window of where the baby is sleeping. In my case its out the front of our unit in the 'common driveway'. I know that this isn't always convenient, but I use the time to pull weeds, write shopping lists (if toddler lets me) or simply play with him and give him some exercise. It takes dedication as I know you want to use that time more effectively by doing chores or cooking blah, but its the only way. Turns out though, that my baby still only sleeps for 45min regardless of inside or outside activity. Ive somehow managed to train my toddler to stay quiet when putting the baby to sleep, and now he sees it as 'helping me' and I try to involve him by letting him shut the door or put on the baby's sleeping bag or a kiss for a nap. Then he stays away until baby is asleep and he knows that it becomes 'his time' and makes a run for his shoes and the door.
Hope it helps.