The diagonal brace is likely overkill and will get in the way of accessing the underside or a shelf should you choose to put one in. The whole bench will likely slide along the floor before it racks should you push hard enough.

Just writing so that other bench builders make that consideration not to shit on your project.

Wood putty won’t add any strength. The crack and piece are large enough where seasonal variation would likely cause the putty to separate from the wood anyway.

I would leave it alone. Given the age of the wood it will likely not crack any further. It has already dried as much as it can.

I use an inshave (aka scorp), compass plane, curved scrapers, and sandpaper for my seats. It’s laborious! Just started on another tonight and of course it has a few stubborn spots.

Good question about the bench. It’s a 5" pine top but it dried funny, curving inward at the top. So I inlaid a maple strip to get the front back to square.

Windows 10 frequently nags you to update to 11. The nags are very intrusive and annoying!