I think this might be the right solution for OP, especially since they don't seem to have had the "experience" I'm sure many of us have had with this "simple" operation. If you are going to do it, them it should be done using a live OS and a full offline system backup, otherwise its very easy to lose a lot of data this way.

However, symlinks like that can make things confusing really quickly. I would encourage anyone using them in this way to establish some easily checked rules and abide by them. For example, maybe you only use symlinks like this in a specific folder such as /home/expanded. You can still have multiple links there like /home/expanded/on5TBdrive or /home/expanded/onPrimarySSD, but it makes it easier to remember, find, and check those locations later.

When you need to know exactly where something is stored, verify a backup, or find data without the symlink, then you will appreciate a set of rules that helps you.

Hard disagree. Resizing partitions is dangerous and difficult for most computer users.

(I'm a sysadmin who does this stuff multiple times a day, so this isn't negative bias)

Symlinking is quick, easy, totally safe. It's one of the best things about linux filesystems. Use it.

The question implies that the OP wants to create one giant filesystem with all of their data on it. This has its own issues, especially if it is in /home. For one, as someone else pointed out, it's fairly difficult to run your system without /home mounted, and that makes it difficult to resize. Sure, you can set up an admin account with it's home in the /root filesystem and then log into that - but that seems to be a lot of work in itself.

If it was me, I'd set up mount points for file systems that make sense. Maybe /data/Photos, or /data/Music, or data/AppData, or whatever. As much as possible, I'd just point whatever software I was using to those new directories to find the data. If that isn't feasible, for whatever reason, then a symbolic link from /home/Photos to /data/Photos will work seamlessly in most cases.

As far as I'm concerned, after administering enterprise systems using Unix going as far back as the early 90's, symbolic links are a key tool in managing disk space that you shouldn't just dismiss because it's "an unnecessary layer of complexity". Having smaller, purpose designed, file systems allows you to manage them better. Sticking everything into /home is probably not the right answer for anyone.

Use a live image with gparted (most normal distros come with it) start it and the gui should be pretty self explanatory.

Use cfdisk and just edit the partitions.

Please note that if you do this without first resizing the filesystems on the partitions, you are very likely to lose data. You cannot safely shrink a mounted partition.

Edit: oh you mean booted from external media, not an online system. Use gparted. https://gparted.org/

Don't do it on a live system.