BACKUP_MacBookAir 256GB_ Vs._iPhonePro Max 265

Hi. recently I am facing data storage issues and what I learn till now that option backup iPhone on local storage on my MacBook Air is useless. If my phone is full back up on my Mac is not possible because there is not enough space on my hard drive. I do search how to change folder for backup - sims that there is no option to change folder to extended HD. As well like restore from backup needs to be done from local folder on Mac HD. with one iPhone in family its complicated but with 5 of them is impossible to work with backups. Files are too big to manage them on MacBook Air. I also have 2 iClouds accounts with 400 GB storage but with allowance to have 6 members in to family with 6 mobiles accounts even that is not enough for smooth operating. So what I guess all of this is made to push purchases of cloud storage. What I do not understand is why even if its not enough storage on my local HD I can not change it to extended one like with photos albums I can choose location of library. And if you have some old backups you want to search for something it's impossible. Copy one by one to local drive restore delete it takes like weeks to do so. If someone manage that through 3-th part software please share with me. Im not sure if im right so any ideas?