How to handle situation where old kernel works, new kernel doesn't?

If I install Kubuntu 20.04, with network disabled, from a USB stick installer that I prepared just after 20.04 was released, then my laptop suspends and wakes ok (Dell Inspiron 3793, Intel i5 Ice Lake, Nvidia MX230).

But if I use the same USB installer with network enabled, or use sudo apt update && sudo apt upgrade after installing, then my laptop will not wake correctly from suspend - I get a black screen.

It is ok if I boot the upgraded installation using the old kernel using the grub menu.

Original kernel: 5.4.0-26 (working) New kernel: 5.4.0-53 (not working)

So short term I need to use the old kernel but I would still like to upgrade. Will the upgrade process ever delete the old kernel automatically? Can I make grub boot with the old kernel by default, so I don't have to go through the menu, without the upgrade process changing it back?

And I probably need to report a kernel bug. But I've looked at the kernel changelogs and there are hundreds of changes netween 5.4.0-26 and -53. (The upgrade included not only new versions of Nvidia drivers (450) but appears to have new versions of the original 440, but the old kernel works with the new 450 drivers, so it would appear to be a kernel issue) So how to proceed?

Update: output of sudo dmidecode -s bios-version is 1.11.0. (This is a new laptop that came with Windows installed; the one thing I did before obliterating Windows is a bios update from Dell's web site).

Update 2: Output of sudo blkid:

/dev/nvme0n1p2: LABEL="SDD 128Gb System" UUID="eecfa2fb-0733-46f5-96a7-e050724ae2a3" TYPE="ext4" PARTUUID="d6f0f6e9-3d9f-48c3-bc1b-709549a864bc"
/dev/nvme0n1p1: UUID="3795-6278" TYPE="vfat" PARTLABEL="EFI System Partition" PARTUUID="292dce4c-e774-4a68-974e-9460f49c3409"
/dev/sda1: LABEL="HDD 1Tb Data" UUID="8dddb3ce-30e8-4356-8b34-62c340b74807" TYPE="ext4" PARTUUID="445e5b9a-cb9f-a04c-b062-67b2b75c4a71"

Output of grep -i swap /etc/fstab:

/swapfile                                 none            swap    sw              0       0

(I don't know why it says that, the GUI Info Centre tool says I have a 2Gb swapfile. But it is not in /etc/fstab - is there another way of configuring a swapfile?)

Output of cat /etc/initramfs-tools/conf.d/resume:

cat: /etc/initramfs-tools/conf.d/resume: No such file or directory

A new 5.4.0.54 kernel came through sudo apt upgrade today, but no change. Nvidia MX230 - I have tried all available drivers presented to me - there were several - and a new one that came through sudo apt upgrade yesterday - but no difference (I have also tried Nuveau with and without nomodeset.) In a previous install on the same laptop while investigating this problem I configured a 20Gb swapfile and I could hibernate and resume ok. It is just resume from suspend that does not work with newer kernels.