I'm not sure I understand honestly. Why would 5th level characters have powerful magic items? What encounters would be too easy compared to starting at 1st level specifically? The random ones? The fixed ones? Admittedly it would take a slightly modified set of encounters if the PC's come in higher level, but I'm not sure I understand the issue.
5th lvl was exaggeration. But at 7th you can get a +2 magic sword that gives desadvantage in half of enemies you'll encounter in underdark, and there is more itens and even blessing to all characters (that work like a stackable magic weapon), PCs can forget that last one for one legendary magic item (ruby spell gem).
Seriously, after lvl 5, the encounters start to get too easy. Random encounters are at max of 4/day, most part 2/day, but the chance that happen is 0,81% and 9% respectively. So, most of the time, party will get 1 encounter day, they can burn all resouces in that one. IIRC biggest xp in random encounter at half part is 2300xp (CR6), one player lvl 6 with full resources can handle this.
Any city the players go last, don't have encounters to match lvl 5 to 7 players:
whorlstone tunnels hardest encounter is cultist hideout, the sum of monsters is 1950xp, should be a 3900xp ecounter by the number of enemies, a hard/deadly to 4 lvl 5, but they all will not attack together since beggining. There isn't no other hard encounter to party, unless they do whorlstone tunnels without rest.
Yestabrod in Neverlight Grove is CR 4, how desapoint this can be for 4 lvl 5?
The Feared Pudding king? another CR4, the royal oozes are another CR4 and one CR1/2, and are separated encounters. biggest xp encounter in blingdenstone? a Solo CR6 creature.
Second half of the adventure is even worse, 5 lvl 11 PCs will defeat a CR22+ demon lord "easily", for some demon lords you just need a warlock with repealling blast, in fact.
Seriously, if you want to challenge PCs, DM's will need to change every encounter, change the number of encounters/day or rest recovery. DM will need to change so many things about encounters after 1/3 of the campaing that really doesn't make difference start at lvl 1 or 5.