WIN10 - LAN file sharing / User permissions

My goal is to create a network share on my main PC that can be accessed from my laptop - but not by other machines/user accounts that are on my LAN.

Set your laptop user as a user on the Windows 10 computer that you are sharing. If you have a Microsoft Account, this step may not be needed. Make certain the users have good passwords.

To set up a user from another computer on a machine:

Use Manage Computer (right click Computer) and select Users and Groups. Add users here and the password the user must use. This is how I do it here.

Share the folder you wish to share to the user above and no one else. Be sure "Everyone" does not have access.

how is it possible that I grant access to myself (local user account on main system), but other systems (external user accounts) on the LAN can map to the drive without any obvious authentication (password)?

First, make sure the Windows 10 machine that you are sharing has a strong password. No password is a security issue. Then make sure (Network and Sharing center, Advanced Settings) that password protected sharing is enforced (enable password protected sharing).

If you do this and also so long as you make sure "Everyone" is not included in Folder sharing permissions, then other users cannot get to your server.

Windows 10 is secure and you would have to permit other users as described above. Otherwise, they cannot access.