Your deck boards are getting near the end of their life with the splitting. If there's no real rot, if it was mine I'd probably try to get it to last a few more years. A good power wash and a solid color stain would likely do the trick. I've heard good things about the Rust-Oleum Restore products that fill the splits if you want to do better.

Anyone have brand recommendations for stain that don't peel terribly? I have looked into the thick deck restore paints, such as Rust-Oleum Restore, and dislike the look, which is why I would just replace boards with splits. I think I can get another 5 years out of my deck.

I pulled one of the split deck boards up and revealed a joist that was rotted and just crumbled when touched. My deck structure is probably more dire than I had previously thought. However, a complete replacement is just not in the cards at this time. Any advice for trying to get my deck safe and usable with just a couple hundred dollars rather than dropping $20k+ on replacing the deck?

Is it accessible under the deck? You could do an ice pick test and sister some good treated lumber next to any rot. Usually the decay starts where wood touches other wood. So if the top 3/4" of a joist is punky, a 2x4 could be sistered up to the joist and attached with a bunch of deck screws. If a joist is halfway gone, you're in for more major repairs.

I spoke with a few pros who said that because my deck is 20 years old, it was made with lumber that wasn't treated and so repair is not advised nor will the workers be insured to work with that old of a deck.

There was treated lumber 20 years ago. If it wasn't treated it would have rotted years ago.

I think the pros who said that meant that it was unlikely that the wood was treated because of how quickly my joists rotted. There have been signs of rot since before I purchased my home in 2014. I just didn't know the rot on the joists was that bad until I pulled a deck board up recently.

Also, they said their insurance wouldn't cover them working on a deck older than 10 years anyway. I know there are higher standards for deck structures now. Most of my neighbors have all already gotten their decks replaced.