Before outsourcing, you need to know what type of book you want to make, the niche, the style, how is it going to be useful to the buyers and why would they buy yours over someone else.
You need to spend some time understanding and finding topics/areas you are happy to work on (you can outsource the interior, but you will still need to spend time researching the niche, the keywords, the title/subtitle).
Planners are always trendy especially in Q4 but you have to be able to stand out in the crowd as a lot of other sellers will be going for the same thing. You have to narrow it down to more specific niches. It could be budget planner, meal planner, yearly planner, monthly planner, weekly planner, 5 years planners etc.. You will also need to make them appeal to a specific niche, will it be for moms who love their pugs, video gamers, etc..
Lets say you want to make a 2022 weekly planner and don't want to make it yourself. You could get one on Canva/Creative Fabrica (be sure to take one that you can edit).
Then from that same template, you could make different versions by changing the fonts, adding images of pugs on every page, adding a floral background, adding the UK holidays,etc.
This way you will have different versions using one template.