Trails games seem to go on sale about once every 3-4 months or so and they all go on sale at the same time. Should be due for another one soon I'd reckon, also something I watch out for lol. They never have massive discounts though, I usually see 33% off, possibly 50% on the oldest ones. Falcom doesn't do bargain bin stuff like Capcom/Ubisoft where they drop to $5-10 eventually, they know fans are dedicated enough to pay up to get the full story.
I've played through CS1 and 95% of CS2. At the very end of CS2 in the epilogue there started to be clear references to the past series set in Crossbell, so I stopped. Decided I might want to play through those two games (starting with Trails from Zero) to better appreciate the CS2 finale. Kinda waiting for a sale on those at this point.
I did go back at one point and play the PSP version of Trails in the Sky on an emulator, but stopped after about 10-15 hours. It was just pretty slow moving and the commitment to play the three Sky games first was pretty overwhelming. Now that there's a remake announced for the Switch I'll probably go with that when it's released.
Really, really loved Cold Steel 1 and 2 and in theory I'd love to play them all to get the whole universe experience but it's just so much. I still might just fold and continue with Cold Steel 3 at some point depending on how I'm feeling.
I have a similar experience with Yakuza. Started with Yakuza 0 and loved it, played through Kiwami 1 and 2. Now I'm going through Yakuza 3 and it's just...okay. But I have 4/5 too and feel obligated to play them though part of me wishes I could just go to the more modern ones. With both series', wish I had gotten into them sooner.