By "HDD" I assume you mean internal storage (no phone has ever had a HDD, or ever will), but to answer some of your questions you need to understand how that is partitioned.
There is a distinction between pre-installed apps, which are in the /system partition, and user-installed apps, app data and app updates, which are in the /data partition. You can't modify the system partition without root, which is why you cannot move or uninstall the bloatware. But conversely the apps themselves are not using any of the user-accessible space, so removing them wouldn't give you any more space. However, any updates to them, and their data, do use your space. So disabling the app, which clears it's data and removes any updates, saves you all of the space you can save - using root to remove them would not give you any more. And in most cases you can do that without root - in principle the only things you can't disable should be truly important apps, though I have known Samsung to make irrelevant junk impossible to disable and have heard of some US carriers being worse (the reference to "forced updates" tells me you have a US carrier handset, since they are the only ones to do that). But for those you can disable there is nothing to be gained from root - just disable them and you are done.
For the rest, whether you can move a user-installed app to SD or not depends on the developer, and not all support it (no system app can be moved, whether rooted or not). I think you can force move user-installed apps if you have root and some other app or mod (root by itself just gives you extra privileges, it doesn't in itself provide the tools to use them). You may be able to move more apps even without root if you just use ADB to change the default install location: the command is something like "adb pm set-install-location n", where n = 0, 1 or 2 (for auto, internal, SD, can't remember which is which), which can be done without root. Note that moving an app only actually moves part of the app and doesn't move the app data, so how much you gain will depend on how the app is structured.
There are other tricks involving partitioning the SD card and using a script or an app like Link2SD to move the apps to the new SD partition. This allows you to move more of the app, and even app data, but I'd do a bit of reading to check compatibility with your Android version, since I've seen a few odd posts that suggest it's not as reliable as it used to be on all devices.
As for making the phone treat the SD as an extension of the internal storage, that's a feature called "adopting" the SD card. This was introduced in Android 6, but not all manufacturers have chosen to include it. I know Samsung specifically don't include it with their flagships (their concern is that it will reduce performance and people will blame Samsung rather than the fact that SD cards are just slower than the internal storage), I don't know about lower-end devices. If you have it you'll find the option in the storage menu to format your card as internal storage - if you do this you won't be able to use the card in any other device (it will be encrypted). If you don't have that option I think you'd need a replacement OS that includes the feature to enable it, i.e. a custom ROM with adoptive storage support, if such a thing exists for that phone. But I'm not an expert with this feature, because it's never appealed to me (as well as being slower SD cards are fragile, and will wear out faster when used that way, so I decided about the time it was announced that I wouldn't want to use this feature).
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