Liverpool haven't made a top class signing for 5 years now so I would say what they are doing isn't working. A lot is living off successful transfer windows from 17-19. What has happened since then?
They have lost Mane, Fab, Bobby, Gini in recent times and none of the replacements have been as good as that team from 18-21.
If Liverpool are not careful they will end up in the wilderness again when all the "winning" mentality players have all left and moved on to pastures new.
I don't agree with your opening paragraph and using the likes of Mane and Firmino as benchmarks is so extreme that it's verging on nonsense. Replicating what Liverpool built with that side was incredibly difficult because that side was so good. Liverpool's ex chief egghead was doing some interviews recently where he stated that Pep's City side is the best club side of all time, better than peak Barca with Messi and he's right. The level they've reached, consistently, for the period they have is unreal and Liverpool went toe to toe with them despite spending a fraction of what they did because our success in the transfer market was ridiculous. Being able to repeat that over and over is nigh on impossible.
We've not reached the level of before in recent years but we've not spent poorly, unlike Chelsea and Utd, we've just not spent as well as we did. As we saw last season, we are still able to put together a nucleus of a squad that can challenge. We ran out of puff and injuries started to catch up with us at the end of the season and the challenge will now be to put that right but I don't think that will be achieved through transfers alone. Yes we could do with a natural top class 6 but I'm struggling to see who is out there that could do that job. Other than that, the only obvious need for the squad imo is another CB but I suspect VVD's age complicates that. Had VVD been 2-3 years younger we could go out and sign a 4th choice option without too much fuss but with VVD needing replacing in the short to medium term, that makes no sense as you'll still be in the market for a CB in 12 months time. Instead the club will want to sign a CB that can be the 4th choice today but a potential starter in the future.
No plan B and also that being the only planned signing is still awful though
If there's no plan B it will be because the club does not believe there's an alternative player out there, available and affordable that's up to the job, not because they didn't bother looking or planning for alternatives. As above, Ian Graham has been doing a few interviews recently talking about recruitment at Liverpool and in one of the interviews I listened to he made the point that the club aren't just looking at players for positions they're actively looking to make signings in but everybody, everywhere. Whether we like it or not, if it proves to be the case that we don't move for anybody after Zubimendi then it's because the club have looked at who is out there and decided the rest are either not good enough, not suited to how we play, too expensive or too big of a gamble.
I'm not convinced he is/was the only planned signing either. We didn't know about Zubimendi until a few days ago. I get the feeling there's still more to come re Gordon and I reckon we'll sign another defender too. I wouldn't rule out a number 6 still either if the club can work out the right deal for the right player as I think that's the one position that Slot will want boxing off ASAP.