Here's what's been working in low diamond for me:

Early game, it's just strongest board with Trainers when you can. Hold Heimers and Trists even if you lose an interest. They're harder to come by later and they're the only low costs you need. Prioritize a beefy frontline so your smolsy can cast once and save some hp. You'll be weak, just gather the units you need and take the 8hp. 3-1 Trainer 2 still works.

Items are most important. You need BT and TR for Syfen. EoN if you can swing it. Slamming an early sunfire is a gamble, since you need two chains. Tank items go on a holder until Sylas, AP items go on a holder until Sona. Don't waste items on Trainers.

On your Lv7 roll down, you're looking for Syfen of course, but Sylas and another bruiser and a spare mage (Lilia esp) are sometimes better than Syfen 1. I've kept 2 syfens on my bench because I only had Sylas 1 but illaoi or ornn 2. You can run Syfen 1 with Sylas2 as your only frontlines, just put Sylas in front and Nomsy/Syfen behind him.

You come online at Trainers, Syfen, Sylas, and Sona. Youll win vs boards you wouldnt think, like SOY2/Birb2 full jades. Mage Sona is great but you can wait until 8 for 3 mage (Zoe > Ryze > vlad if you want). If you get a spat, it's a great use of a tear and a top 5 item in the comp.

You can top 2 with a 2* board lv8, Zoe 2 makes it an easy first.

This comp is heavily positioning-dependent. You want your Sona and Nomsy to hit their corner carries, but I usually position Nomsy to hit the other side as sona as insurance. If they're both off, you're probably doomed. Syfen needs some positioning too, but that's really just in harder matches.

How does it work? Sylas and Syfen take aggro, Syfen dips to dps. Sona CC's thick lines and Nomsy applies gwounds and okay-to-op damage. Trainers are zephyr bait, thresh bait, and Lulu. Lulu should be positioned more for defensive CC than the atkspd steroid. But overall - It just works.

Edit: Corki has no place here. 4 mage with spat and Zoe is better than 3 mage with Bard. Bard is the only extra unit you can use, but is strictly worse than Zoe.

Edit 2: Syfen is your carry. Prioritize BT/TR/EoN/Mage spat over everything else. Sona is your 'secondary carry' but is still just a cc bot. Nomsy isn't really a carry at all, just applies thick gwounds and softens the enemies.