Be warned even with apt pinning it could turn your package management/system into chaos. No warranty from me.
For security install ppa-purge first. If something goes bad you can try revert/delete packages with

sudo ppa-purge system76-dev/stable 

another way for be prepared is

/etc/apt/preferences.d/focal.pref 

Package: *

Pin: release n=focal

 

Pin-Priority: 1001

for this in

/etc/apt/sources.list.d/system76-dev-ubuntu-stable-focal.list

the entries has to be comment out.

sudo apt full-upgrade 

Reading package lists... Done

Building dependency tree       

Reading state information... Done

Calculating upgrade... Done

The following packages will be upgraded:

  python3-distupgrade ubuntu-release-upgrader-core ubuntu-release-upgrader-qt

The following packages will be DOWNGRADED:

  accountsservice libaccountsservice0 linux-firmware linux-generic linux-headers-generic linux-image-generic linux-libc-dev ubuntu-drivers-common

  virtualbox virtualbox-dkms virtualbox-qt

3 upgraded, 0 newly installed, 11 downgraded, 0 to remove and 0 not upgraded.

Need to get 43,9 MB/145 MB of archives.

After this operation, 50,5 MB disk space will be freed.

Pinning for kernel

/etc/apt/preferences.d/kernel.pref 

# Kernel pinnen 



Package: linux-generic

Pin: origin security.ubuntu.com

Pin-Priority:1001



Package: linux-generic

Pin: origin archive.ubuntu.com

Pin-Priority: 1001



Package: linux-image-generic

Pin: origin security.ubuntu.com

Pin-Priority:1001



Package: linux-image-generic

Pin: origin archive.ubuntu.com

Pin-Priority:1001





Package: linux-headers-generic

Pin: origin archive.ubuntu.com

Pin-Priority: 1001



Package: linux-headers-generic

Pin: origin security.ubuntu.com

Pin-Priority: 1001 





Package: *

Pin: origin ppa.launchpad.net

Pin-Priority: 300

Packages for kernel from origin archive.ubuntu.com/security.ubuntu.com are preffered (1001)

sudo apt full-upgrade 

Reading package lists... Done

Building dependency tree       

Reading state information... Done

Calculating upgrade... Done

The following packages will be DOWNGRADED:

  linux-generic linux-headers-generic linux-image-generic

0 upgraded, 0 newly installed, 3 downgraded, 0 to remove and 0 not upgraded.

Need to get 0 B/7.028 B of archives.

After this operation, 630 kB disk space will be freed.

Do you want to continue? [Y/n] y

dpkg: warning: downgrading linux-generic from 5.8.0.7630.32~1605108853~20.04~8bcf10e~dev to 5.4.0.54.57

(Reading database ... 291830 files and directories currently installed.)

Preparing to unpack .../linux-generic_5.4.0.54.57_amd64.deb ...

Unpacking linux-generic (5.4.0.54.57) over (5.8.0.7630.32~1605108853~20.04~8bcf10e~dev) ...

dpkg: warning: downgrading linux-image-generic from 5.8.0.7630.32~1605108853~20.04~8bcf10e~dev to 5.4.0.54.57

Preparing to unpack .../linux-image-generic_5.4.0.54.57_amd64.deb ...

Unpacking linux-image-generic (5.4.0.54.57) over (5.8.0.7630.32~1605108853~20.04~8bcf10e~dev) ...

dpkg: warning: downgrading linux-headers-generic from 5.8.0.7630.32~1605108853~20.04~8bcf10e~dev to 5.4.0.54.57

Preparing to unpack .../linux-headers-generic_5.4.0.54.57_amd64.deb ...

Unpacking linux-headers-generic (5.4.0.54.57) over (5.8.0.7630.32~1605108853~20.04~8bcf10e~dev) ...

Setting up linux-image-generic (5.4.0.54.57) ...

Setting up linux-headers-generic (5.4.0.54.57) ...

Setting up linux-generic (5.4.0.54.57) ...

you get for all other packages from your ppa updates for installt packages.

Note remaining kernel/headermodules packages from ppa you have to purge on your own.

You can do it on your own risk. Really. Be warned

Better is to remove the ppa with ppa-purge. Remove all apt pinning files.

Reactivate the ppa again.
then create

/etc/apt/preferences.d/system76.pref 

# pinning für system-dev

Package: *

Pin: release o=LP-PPA-system76-dev-stable,a=focal

Pin-Priority: 300

If you need a particular package from ppa you can install it like this.

apt-get -s install -t focal linux-firmware

NOTE: This is only a simulation!

      apt-get needs root privileges for real execution.

      Keep also in mind that locking is deactivated,

      so don't depend on the relevance to the real current situation!

Reading package lists... Done

Building dependency tree       

Reading state information... Done

The following packages will be upgraded:

  linux-firmware

1 upgraded, 0 newly installed, 0 to remove and 10 not upgraded.

Inst linux-firmware [1.187.4] (1.190.1+system76~1605123765~20.04~3894207~dev System76 Stable PPA:20.04/focal [all])

Conf linux-firmware (1.190.1+system76~1605123765~20.04~3894207~dev System76 Stable PPA:20.04/focal [all])

Another example

apt-get -s install -t focal accountsservice

NOTE: This is only a simulation!

      apt-get needs root privileges for real execution.

      Keep also in mind that locking is deactivated,

      so don't depend on the relevance to the real current situation!

Reading package lists... Done

Building dependency tree       

Reading state information... Done

The following additional packages will be installed:

  libaccountsservice0

Suggested packages:

  gnome-control-center

The following packages will be upgraded:

  accountsservice libaccountsservice0

2 upgraded, 0 newly installed, 0 to remove and 9 not upgraded.

Inst accountsservice [0.6.55-0ubuntu12~20.04.4] (0.6.55-0ubuntu13.2pop0~1605745773~20.04~d9482b1~dev System76 Stable PPA:20.04/focal [amd64]) []

Inst libaccountsservice0 [0.6.55-0ubuntu12~20.04.4] (0.6.55-0ubuntu13.2pop0~1605745773~20.04~d9482b1~dev System76 Stable PPA:20.04/focal [amd64])

Conf accountsservice (0.6.55-0ubuntu13.2pop0~1605745773~20.04~d9482b1~dev System76 Stable PPA:20.04/focal [amd64])

Conf libaccountsservice0 (0.6.55-0ubuntu13.2pop0~1605745773~20.04~d9482b1~dev System76 Stable PPA:20.04/focal [amd64])