Destroyer?

I've finished the game but have 3 tasks unfinished. Haven't completed Aid APEX, Frequency, and Task Data Remnant Cores. Heleus galaxy says I'm 98% complete and I assume that's due to those 3 unfinished tasks because all the systems are 100% complete. So I was wondering is there anywhere I can go to scan a destroyer? I've done it in a previous gamesave so that's how I know I missed the scan of a destroyer. But just wondering if there's just one walking around on a planet somewhere. Frequency I can't finish due to a glitch in the game. Also I missed a couple or remnant cores in the vaults. I definitely missed one on Voeld. Is there anyway to go back in and get them without using an old gamesave?