"The untrammeled power... of Johnson and Truss" - is that the same power that stopped Johnson from trying to get Brexit done? Are you saying that FTTP was responsible for having a Brexit vote, or do you mean it caused the problems of trying to implement the will of the people?
Johnson "got Brexit done" (in quotes because there are in fact many unresolved issues) when he won an 80 seat majority in 2019, even though a majority of the public were against it by then and a majority also voted for parties that favoured a second referendum.
Under FPTP you can only really ever have two big parties that have a chance to win power, occasionally as a coalition as in 2010 and 2017. UKIP in 2015, then the Brexit party in 2019 and Reform in 2024, have tried to force the Tory party to the right by threatening to stand against it, split the right-wing vote and cause the Tories to lose a lot of seats to Labour. In 2019 they withdrew this threat and made deal not to stand in Tory seats, this being one of the main reasons for the majority. Under a proportional system they probably couldn't have done this, unless they happened to hold the balance of power as the DUP did under FPTP in 2017. One of the main criticisms of proportional systems is that small parties have too much influence if they happen to hold the balance of power, but these examples show it can happen under FPTP too.
Indeed, so I am not sure what you mean by "would have powers similar to the Lords today, but would be less afraid to use them"
It means I'm thinking that the formal powers of the Lords would remain similar to what they are now, but its members would be chosen in a very different way. But at present they are reluctant to step in, particularly if the legislation they are considering is a manifesto commitment, because they know they aren't democratically elected and if they attempt to block what might be seen as the will of the people then they will lost what legitimacy they have. If the Lords was also democratic then this concern would disappear.