Help Tech-Savvy NOOB get started

I have an old Sony Xperia M C1905 that I want to root/load a custom, cut-down ROM to free up space for the Apps that seem to be growing with every update. I am a NOOB (as you say) to installing custom F/W on my phone, BUT I am tech-savvy. What I need is access to a set of instructions/tools that I can use with my phone and PC combination. I realise that this is an old phone and you are trying to help, but for me it is frustrating when all the links to tools from the threads seem to be dead or don't work. Please can someone have a read and let me know a way to do this that has a set of working tools. Thanks

My PC is a MS Windows 10 laptop – If necessary, I could resurrect an old laptop and install Linux on it, if that is a simpler approach.
I have had a look at what is available and will probably load the ROM from:
https://xdaforums.com/xperia-m/orig-development/rom-slim6-marshmallow-t3418258
What I think I need to do (and have tried to start) is:
1. Root the phone.
2. Backup the current ROM.
3. Unlock the boot-loader.
4. Load the Slim6 ROM.
To do 1. I have tried two methods, the first recommended from the backup thread and a second found by searching.
i. Easy Rooting toolkit: https://xdaforums.com/showthread.php?t=2292598 but the download link for DooMLoRD_Easy-Rooting-Toolkit_v17_perf-event-exploit.zip. The Download site for this tool tells me (eventually) that the file has expired or has been deleted by the owner - so i can't get this tool. Any Ideas How to get this?
ii. Also tried: https://xdaforums.com/showthread.php?t=2529561 but for me, the cydia impactor tool does not see my phone or have any options. Yes, I have enabled - and allowed USB debug mode, before you ask. And yes the PC can see the phone so the connection is good.
To do 2. I am proposing to use the instructions from this thread: http://xdaforums.com/showthread.php?t=2292598 Please confirm this is still valid.
iii. [Update] Also tried rooting by using this thread: https://xdaforums.com/showthread.php?t=2457174 but this requires Framaroot 1.5.3, When I try and download this, the download site says: "The file link that you requested is not valid. Please contact link publisher or try to make a search."
Update 2. Managed to get a download of Framaroot, installed, but does not work for Xperia M with Android 4.3. Any idea of a way forward?

To do 3. I am proposing to use: https://xdaforums.com/showthread.php?t=2484622 Please confirm this thread is still valid - and the best approach.
To do 4. https://xdaforums.com/xperia-m/orig-development/rom-slim6-marshmallow-t3418258 - Please let me know if there is a more appropriate ROM to install to free up some space.

What about backing up the current ROME?

Thanks for your reply, but to my understanding this just does 2. & 3. from my list. Most of the ROM installation guides say to take a backup of the current ROM so that you can restore the original system.
To do this I need to get root access with the current build. Is this now impossible?
Maybe I just need to bite the bullet and just not bother with the backup.

Thanks for the confirmation of the process.

Iovyroot is not a root method I have tried. I will Investigate tomorrow.

I will post back here when I have tried it.

I have tried a couple of versions of Framaroot, one recommended version for the phone, and the latest version I could find. Neither worked.

Is nidhoegg mistaken?

Please can you confirm that the TA partition will Be NOT be formatted when I unlock the boot loader?

My understanding is that this contains various sets of system data including: S/W signatures for the Sony Apps (if I want to restore the Sony ROM to the phone); calibration data of the various devices on the phone - accelerometer etc. So surely, even if just for the calibration data, I would not want to lose this data even if I loaded a custom ROM - I have no way of re-calibrating the devices, so would need the factory acceptance calibration values.
Am I mistaken?

Are you saying that all the threads from when this was the current phone are wrong, and I don't need to backup this partition?
TA = Trim Area; as well as holding the DRM Keys/S/W signatures, my understanding is that at contains Trim data to adjust the various sensors on the phone to read accurately. I have been led to believe that these are written during the automated Factory Acceptance Testing to adjust for small differences in the components on the phone, and are unique to the device. Surely if I lose this data the sensors will not read accurately.

Is this wrong?