I think
@Dier Hard's point about a clear footballing philosophy and style of play is the single most important and most likely factor to improve a club's chances of getting more transfers right than wrong. For starters, it makes the scouts' job a hell of a lot easier if they have clarity as to what profile of player they should be looking for. They will also look at certain teams in certain leagues who play a similar style, as those players are more likely to adapt tactically.
One of the reasons our squad is such a mess at the moment is not because we haven't spent money. We've spent plenty, but under 4 different managers who use three different formations and (broadly speaking) tactical systems. And so while in all honesty I couldn't see them ever flourishing for us, I do have a degree of sympathy for the likes of Sess, Tanguy, Doherty and Lo Celso who were obviously signed by a specific manager who valued their profiles with a specific vision in mind, only for said manager to be gone within 3-6 months of their signing to be replaced (or not) with a new manager who values different profiles.
I think this is what helped our early windows under Poch be so successful. We had a clear, unified vision from top to bottom, a strong network led by Mitchell and a desire to back the manager. It felt like we were hand-picking players for pre-determined roles, like a good casting director selecting a cast for a script. Dele, Dier, Toby, Son, Davies, Wanyama and Trippier all came in and adapted very, very quickly. If anything, we were perhaps frustrated with Poch for his insistence that new signings had to undergo 3 months of his strict fitness regimen before they got regular first team football.
Things started unravelling when due to the new stadium we tried doing things on the cheap - Sissoko, Aurier, N'Koudou and Lucas spring to mind. Then we stopped spending entirely. Then we started again in a serious way but sacked the manager who'd spent it 3 months later and thus began 4 years of patchwork squad building.
In the years 2014 - 2019, the only true 'flops' were Sanchez and Janssen - and in all honesty he latter wasn't even that expensive, I'm just counting players who were more than £20m which even today is a significant sum (Lucas is redeemed thanks to *that* night in 'Dam).
In the years 2019 - 2023, we'd count Sessegnon, Ndombele, Lo Celso, Bergwijn, Richarlison and Royal...and the jury is very much out on Porro although time is on his side.