I have another question.
1 - Would it be safe to re-size the sdb 1 partition, like maybe down to about 20 GiB?
No point, currently, Charlie
Wouldn't make a difference, you can't use the redeemed space.
Let me explain.
Under BIOS-MBR, there is the 4-partition Rule. This was implemented with MS-DOS by you know who, and has nothing to do with any blame on LInux.
With that Rule, you can have up to 4 Primary Partitions - a couple of them might have Windows stuff on them, or not, one might be Swap if you used it.
If you want to go past the 4-limit, you have to create an Extended Partition, which takes up next to no space itself, but acts as an umbrella over the top of Virtual Partitions you can create to hold - of course, more Linux.
So you would make /dev/sda4 typically (dev/sdb4 in your case) the Extended, and then the first Virtual Partition which would act as Root / for another LInux would be /dev/sda5.
Swap, when used in a Partition, does not care if it is on a Primary Partition or a Virtual Partition, so it could be on /dev/sda2 or 3, or on /dev/sda5 or more.
What
you have done, and this is a simple (not quite) error to make, is to jump in early and make your
/dev/sdb2 the Extended.
Linux follows what it is told to do with the Rule, and so your first Virtual Partition, /dev/sdb5 which houses Elementary 5.0 'Juno', is number just that - 5.
You lose the opportunity to make use of /dev/sdb3 and /dev/sdb4 completely.
The only way to remedy that would be to delete the Linuxes from /dev/sdb5 to 10, then remove the Extended Partition /dev/sdb2 and put stuff on /dev/sdb2 and /dev/sdb3 and make /dev/sdb4 your Extended and then work from there with a new /dev/sdb5.
Bottom line for your question is that you can resize /dev/sdb1 using the GParted in another Distro, but you won't gain anything of useful value.
Doesn't matter in the long term if we change the setup to UEFI-GPT.
Gotta scoot and cook tea.
Later
Wiz