Garage Roof Sagging...How to Reinforce?

Hello folks...long time lurker so I am new to posting here. I recently bought a home in June 2024 and I have some questions about my garage roof for folks more knowledgeable than me. The house was built in 1985, so my detached garage was likely also built around then. However, I do not think the garage was built to code. From neighbors I have learned the original owners were pretty mechanically minded, so they likely just built the garage themselves after having the slab poured. I say this just to provide a little background on the situation.

The roof has about 5 inches of sag in the middle. This is right where the ridge beam is cracked and it looks like they sistered on some support pieces to provide strength (pictured). I have been monitoring the roof for close to a year now since moving in, including through a Canadian winter, and it does not look like there has been any further sag since we took ownership. As well, walking on top of the roof reveals no soft spots. There are a few leaky spots, but nothing you wouldn't expect for a nearly 50 year old structure. The decking on the front half of the roof was replaced in 2016 which you can tell from the stamps on the material, and I'm assuming the shingles would have been replaced then as well.

Taking a look inside you can see that there are several lower collar ties warping and twisting, causing nails to be pulled out. You can tell the previous owner tried to nail them back in several times only for them to come back out. There used to be a woodstove in the garage, so I think the warping was due to the rafters drying out over and over in a humid climate. By count, there are 12 sets of rafters and 7 of them have warped collar ties with nails pulled out.

So, my question to you all is what do I do to strengthen this roof? Should I put a support beam in the center where the ridge beam is cracked? How do I fix the warped collar ties? I am pretty mechanically minded and have tackled projects such as kitchen and bath renovations by myself, so I would like to try to do as much of the work myself that I can. However, I know that sometimes it's just best to call in the pro's. Thank you all in advance for sharing your knowledge and expertise!

Photos Here:
https://imgur.com/gallery/garage-roof-ERnNWRl