Splitting/merging in iTerm kind of works but it never felt intuitive to me.
With that in mind, I built something to suit my workflow:
Waffle is a native macOS terminal (Built on Miguel de Icaza's SwiftTerm) that tiles your sessions into an auto-scaling grid automatically. 1 session is fullscreen, 2 is side by side, 4 is 2x2, 9 is 3x3. Open a terminal, it joins the grid. Close one, the grid rebalances. No splitting, no config.
I've been using it a lot recently and one thing I've found really useful is that sessions detect which repo they're in and group accordingly. Each project gets a distinct colour. Cmd+[ and Cmd+] flip between groups. If you have three repos open across eight terminals, you can filter to just one project's sessions instantly. Also, no accidentally closing a window with CMD-W as it gives you a confirmation and requires a second CMD-W to close.
Honestly, if you live in tmux, this probably isn't for you but it's really helped to speed up my workflow.
Other things: It comes with a handful of themes (and has support for iTerm themes), bundled JetBrains mono, has keyboard shortcuts for everything. Free, no account, opt-in analytics only. macOS 14+.
There's a demo on the landing page if you want to see it in action.