Looking item by item, I found following and very old forum post on how to allow lead to contact conversion without forcing account creation:
https://community.suitecrm.com/t/suite-crm-free-version-are-these-possibilities-available/79517/4
I wonder if I can do the same during contract creation (in order not to force account association with new contract).
Also, this method includes changes in php code, which means that with any new version of suitecrm this change will be overwritten and I wonder if there is a smarter (and upgrade-proof) way of doing this (this posts are old, solution is 10 years old)?