I do think that another thing to keep in mind is that the imbalance means less with a prepared caster than a known one. Yes, the Green Knight gets a huge arsenal of additional spells, but how many of those are going to edge out staples from the herald list? I'm currently playing a level 8 O5E paladin in one of the games I'm in and you really just don't get that many spells prepared as a secondary caster. Let's say I've decided to convert that PC to LU, and futhermore I'm fortunate and have a Charisma modifier of +4 (BTW, the original PC has a +3, so I'm giving myself an extra point here); that's a grand total of 8 spells of 1st and second level.
Off just the base herald list of from-the-O5E-SRD-spells, I'd consider shield of faith, divine favor, bless, and lesser restoration to be absolute prepare-these-every-day essentials.
So that's 4 spells to pick from. Looking at the new spells from A5e, I really want calculated retribution and force of will. Now we're at 6.
So as an 8th level character, I have 6 prepared spells out of 8 decided before I even start looking at what I get from my subclass.
YMMV, of course, but it's like a buffet. You technically have all these options, but your stomach can still only hold so much.