We are in the realm of Personalization; opinions.
You can have everything on One single partition or you can spread everything out on several partitions if you are using EFI with GPT. Opinions can be strong and are often well supported. Even a very well supported opinion must apply to your needs to be relevant.
It's like getting into Dating. Everyone has lots of opinions and lots of advice and if any of it was worth anything; the world wouldn't be such a mess.
Application matters. And while a person using SETI@home might have certain needs for network speed, a novelist in a lighthouse might have utterly different priorities.
Which is why I think you cannot find a definitive answer on the internet as to what partition sizes you need. I do not use Flatpak or Snap at all and the first thing I do when I install the OS is remove those. Entirely. You make a very valid point about how sandboxed and oft-updated applications can become Room Hogs, complete with some guy on the couch- Something I forgot about as I do not use either. That being said, I still think 64gigs really should be plenty.
But you - as an individual with your own priorities - may feel that 150gigs is more than enough for your /Home partition. In which case, you could assign all the reminder to / containing /var and have that peace of mind. Even if it was far more than you need it to be, as long as you are comfortable and not stressed; you are suiting your computer to you.
"Swap Partition" is different from Swap File. Swap partition is only really necessary if enabling hibernation. A Swap File can be used on any partition and is easy to set up or modify.
In my AnnLanders opinion, it is unnecessary to create a separate partition for /var simply because the same limit applies. Adding a partition creates complexity and more moving parts. This applies to /boot, as well.
It can be helpful to have /boot on a separate partition if you are running LVM with full disk encryption or dual booting windows. Some say it allows the machine to be bootable even if part of the drive is corrupted. But I say if you are booting a corrupted drive, you already have a problem and are likely in the process of creating new ones.
We see again where application matters. If you are not an agent carrying your notebook in a briefcase with full disk encryption Or a person that never does backups and worries about hdd failure (which is a bit of its own magic...) then you would only be needlessly complicating your setup by building groups of separate partitions. And I am sure a quick net search would yield plenty of opinions stating strongly that they should all be on separate partitions. With iron chains and aircraft radar, too.
Partition Options during installation:
Let's say you would like to create your new installation with a separate partition for swap, /var, /boot and /home. When you select the Something Else option in installation and open the ubiquity gparted application, you will need to modify any existing partitions to claim free space, then adjust each to be the size you want, then set the installer to recognize the partitions for each directory. I would not really call it evident in anyway as nothing on the screen will advise you on what to do. It only displays the values. You would need to have a Plan (Perhaps a written chart on a piece of notepaper), following each step and checking your work to ensure no mistakes as you go.
On Zorin OS 16: It does come with bells and whistles. 
Jelly Mode was fun to look at. For a minute.
I have not used it since. Not knocking it.
If there was a reason I would encourage a person to up their installation to Zorin 16...
- Speed and performance. Utterly unmatched. If you want your computer to be faster than Ubuntu 20.04 or Linux Mint, this will do it, easy-peasy. I do not know if this falls under "lean" or not, but it is a greyhound.
- Being up to date on more current editions and kernels. Only you can decide how well this applies to you. If you are like me, then you do not get hot in the face over the Latest and Greatest. More often than not, it is the old, with a shiny wax layer. But for some packages, a later update really brings very useful tools. For me, Cad, Gimp and Blender all come to mind where this applies. Installing Z16 now may prevent you having to do a full installation later...
But, Zorin OS15.3 is still LTS, still supported, still great. Only you can weigh it all and Convince Yourself of what you want.
Hopefully my lighthouse novel here is helpful, if a bit lacking in clear specifics and opinionated.
