Those numbers are disingenuous because some of those attacking players were funded by the sale of other attacking players. For example you've counted £30m each for Sanchez and Mkhitaryan - how does that work when they were both a direct swap?
Once you take out the numbers money we got back for Depay, Di Maria, Mkhitaryan and Lukaku that net investment comes to about £286m in attack. And that's still counting Pogba as an attacker when he's spent the vast majority of his time here in a midfield two.
So that means our investment in attack (which is by far the most valuable part of a team) is actually only slightly more than defence using your own numbers for defence.
Then we have to consider the other aspects too like Valencia retiring, Young being old. Shaw breaking his leg and being very injury prone, Bailly not able to stay fit for more than 2 games, same with Rojo and Jones. We had 1 reliable defender and that was Smalling. Defense needed investment as we didn't have RB and reliable CB who can stay fit.
We also have to consider how much market has changed, 60 million we spent on Di Maria was huge fee back then but now every decent player is rated at higher fee.
We broke record for Maguire among defenders but when we signed we broke records for Mata, Di Maria, Pogba, Lukaku (for ManUtd, world record, PL striker record) and also broke records for wages paid in PL.
We need investment in all areas as we more or less purged the title winning squad.
Its not as if we ignored attacking position either, We signed Mata in 2013-14 when we had RVP, Rooney, Kagawa, Nani, Valencia and others.
In 2014-15 we added Falcao, Di Maria.
In 2015-16 we added Martial, Depay, promoted Rashford.
In 2016-17 we added Zlatan, Pogba, Mkhi
In 2017-18 we added Lukaku and Sanchez
In 2018-19 and 2019-20 we added only James and Ighalo
In every season we added attacking players and sold few too. We score few goals not because we didn't have attackers or ignored that positions, we did as we didn't have balanced team (thanks to managers preferences and their ignorance for right wing position) and poor coaches.
Edit: Btw we should have signed 1 or 2 attackers in this summer but on a whole we haven't ignored attacking positions.