Why do you think United will get better? I am a United fan myself, but I think it will be difficult even to keep the third place, and even more difficult to climb higher. Buying Mount is good, buying Onana is just a replacement for De Gea. I don't see any major enhancements. If United got Kane or Osimhen, you could say that they are getting stronger, but in my opinion now +- the same level.
I see you want United to sign a striker. Kane is interested in move to Bayern and highly doubt Spurs are gonna let him go to their rival and as for Osimhen he is not gonna move this season so seems like it is hard for United to sign any striker as you mentioned. But they actually are interested in Rasmus Højlund (wonderkid from Atlanta)
[1],
so what do you think about this? Asking price around 65~70m Euro and talks are very advanced right now with personal terms agreed with player and United trying to find a proper fee to sign him.
References :
[1] https://twitter.com/FabrizioRomano/status/1681262926810824705