14 across: Career shift nearly converted to chase rainbows? (3-4)
Excellent direct clue: career shift nearly converted = fly fisht nearly = fly fish = to chase rainbows.
19 across: Rose dominating? Ring 501-150 (6)
Excellent placement of the question mark: Ring 501-150 = L I one L = Lionel = Rose dominating ring.
20 across: Tiny blonde bombshell whose quintet was celebrated (4, 6)
Another brilliant direct clue: tiny blonde bombshell = Enid Blyton = whose quintet was celebrated.
5 down: Feller for Spooner’s rational task (8)
Spooner’s are always fun. This one is no exception: Spooner’s rational task = Spooner’s sane chore = chainsaw = feller.
23 across: Sharply returned rose-wreathed cake (8)
Fantastic: rose-wreathed cake = red wreathed torte = retorted = sharply returned.
15 down: _ETTLE T__ BI__? (5, 3)
Just genius: the missing letters are shell, so shell out = settle the bill = _ettle t__ bi__.
16 across: Flashier subhead by show-off? (7)
A risque piece of DA magic: subhead by show-off = s by wanker = swanker = flashier.