most split ergos will be 5 rows or less, so no dedicated f-row. BFO is about the only one i can think of that's 6+ rows.
getting used to Fn +3 = F3 is probably one of the easies layer things to get used to. it's not nearly as bad as it sounds.
i spend all day every day in design programs and do more shortcuts/command combinations than actual typing, and most of these include F-keys.
are you wanting to stay staggered or are you also looking at ortho? (BFO is ortho)
if you do go ortho, and want to keep a "normal number row" i'd suggest noting smaller than a pair of 5x7s (full matrix of 5x14). this will give you a full number row.
keeb.io's viterbi is one option. i love mine, it's a rev.1 and been using it for years. i went with the viterbi because i was already used to the Fn F-keys coming from a 60% board but didn't want to go to the more common (smaller) 5x6 grids.
split stagger... there are a few options that you've already mentioned, but not many more (if any)
just saw this... looks like they might have another split stagger coming out... called the
KBO-5000 "based off of the G80-5000"
yes, OEMs do funny things with caps that are not compatible with most keysets usually because that is not their focus. or you can support strange layouts but you'd have to spend a boatload for additional compatibility kits. this is part of the reason i also went to blank keycaps. with blanks, you learn how to touch type real quick.
if you wanted to do a one-off and 3d print a shell & handwire, you could do that too if you're not finding what you want. it's not terribly difficult to handwire and i've helped a couple here on the forum with firmware & such.