Personally, I’d definitely revive the old account. Getting started is the hardest part, and with a 15k account to jump in with you have a big advantage in making those brands you want to work with think you are legit and having potential new followers perceive your account as an industry authority.
Engagement comes down to a LOT of factors, and some of it is just genre of content, how you use or don’t use hashtags, etc. The algorithm is just one piece of why a post might get the level of engagement it gets.
If you restart it, though, archive- don’t delete- old posts, and archive them in batches of 4-6. Totally setting fire.to all your old content to restart can be a red flag to IG.