Really struggling with updating an old D5 website

4 years ago, I built a small website using Drupal 5.7. I added Image, and Image Assist modules, as well as the TinyMCE editor (and CAPTCHA), so the main content could be updated like a blog. It's been working fine ever since, but I have never done any code updates.

I've recently transferred this website to my shared server, and I am increasingly concerned about its update status (i.e. it may be vulnerable). So, after a long break from Drupal, I looked into updating the code.

What a nightmare! Apparently, we're on version 7 now, and all the separate packages need updating incrementally. Okay, so I get started, my first task being updating all the modules...

Apparently, TinyMCE is superceded by WYSIWYG, and I need to update Image/Image Assist to version 2. But, although it's not too difficult to install WYSIWYG, any editor I install refuses to work with Image Assist, and updates to Image, Image Assist, and JQuery modules cause problems with the images in existing pages.

I've tried different combinations of updates, restored the database several times, and reverted different combinations of modules to their original versions, and that's even without attempting a Drupal core update. 24 hours later, I'm nearly ready to give up :-(

Can anyone give me advice on the best way to bring the code up to date, please?

Drupal 5.7
Image 5.x-1.x-dev
Image Assist 5.x-1.x-dev
TinyMCE 5.x-1.x-dev
CAPTCHA 5.x-3.2
jQuery Update 5.x-2.0