When is max investment needed for gen 1-2 units in AR?

Context: took 22 months away from game, missed a lot of resources (feathers, dew, GHB units, coins, flowers, etc.etc.etc.) and so I'm very resource constrained. I have clawed my way back to tier 22-4 (22 when rage- or stupid-failing, like forgetting my escape ladder last week) in AR.

I am trying to set my 3*-4* investment strategy and am disappointed at the dearth of options that have emerged in the last 2 years. Most of the better units are gen 1 or 2, mostly because of great refines, but it feels like in most cases the builds only work with +10 merge and max flowers to keep up stat-wise with a new 5* fresh off summoning.

Edit: there's a lot of cool Grail units, but having missed the GHB or TT events, it'll cost me a ton more Grails to get to +10 especially as summons 9 and 10 are multiples more expensive than the first two.

Am I suffering from showcase envy? In your experience, at how many merges and flowers is an old unit "good enough" for mid-tier AR? Or, with limited resources, should I focus on +10 demotes from new banners?

(Somewhat related, it's not mathematically possible for all gen 1 units to get a Resplendent version, is it? Considering it works like +5 merges, it can make a difference, e.g. my Cordelia can get to +15 but my Est cannot).