Depends When you say Start-up what is the age of the company? - If it's Under 3 Years - Then Succession planning won't be possible - But companies is 3-5 Years Age should Initiate succession planning by Strengthening their 2nd in line (i.e Founding Team -2) level of employees
with regards to employee development, companies at any age can start this - It can start with smaller & cost effective initiative's like procuring certain books & starting book clubs - Inculcating Product Knowledge - wherein Product owners can take certain sessions & impart the product knowledge - because it's actually important for folks working in the company to know their product or services at a high level.. & once you have a budget then ofcourse investing in trainer led trainings & promoting certifications