I owned the Diamondback 15’s and sold them, not because they sucked, but because I Just didn’t use them like I thought I would. They were always the first optic left at home and the last one packed. That primarily is tied to my hunting style and areas primarily, but I’m not sure I would buy them again.
I would upgrade your chest binos, but I would make a bigger quality jump than to a monarch 7. I’ve not spent a ton of extended time behind Nikon binos, but the little time I have spent behind them has left me underwhelmed. There’s better $5-$700 binos on the market than the Monarch 7, so I’d say update your chest binos after doing some extensive comparisons and testing of them side by side. You’ve got some time to make the purchase, so see if branching out to something else leads to you finding something you’ll like even more. Just my 2 cents from a guy who hasn’t ever owned Nikon glass admittedly, though that also means I’ve always picked something else over them for a reason haha.
Also, no matter what you purchase, pair it with a good tripod and mount. It’ll make any optic you purchase a better tool!