Here are my general thoughts, mostly about matching sibling names:
Axel- This doesn’t seem to fit as nicely with [name_m]Daniel[/name_m] and [name_f]Eleanor[/name_f]- it doesn’t fit the gentle, dignified, and timeless vibe those have. It’s a nice name, but I prefer some of your other choices.
Credence- I really like this name, but I don’t have the same feeling about it as [name_m]Daniel[/name_m] and [name_f]Eleanor[/name_f]. It feels more upright and old-fashioned. I’m also not sure if there’s an intuitive nickname, which [name_m]Dan[/name_m] and [name_f]Nell[/name_f] seem to be. I think this would be excellent in the middle spot!
Elias- [name_u]Elias[/name_u] is a great choice! It has a lovely sound and has the same feeling as the siblings of an old name that continues to be used and feels new. I also really like the fact that it shares letters with [name_m]Daniel[/name_m] and a beginning sound with [name_f]Eleanor[/name_f] as a subtle link between the siblings. However, if the double [name_f]El[/name_f] sound bothers you, maybe this isn’t the right name.
Henry- I do like this with your kids’ names. However, I think it would match well in a different way than many on this list; instead of being a softer name (often with Biblical origins), it has a more upper crust feeling. I guess what I’m saying is that your sibset could match well in totally different ways depending on the name you pick!
Harvey- This doesn’t feel quite as timeless as his siblings’ name’s; while this isn’t a deal-breaker, it isn’t what I would choose from this list.
Isaac- I think this is another excellent match! It has the same old but fresh feeling and gentle strength. I also like that it has a unique beginning and ending letter from the siblings.
Josiah- [name_m]Josiah[/name_m] sounds great with this sibset! He seems to be a stylistic match and the sounds are very appealing with sibling names. This is another fresh Biblical name!
Jordan- I like this name, but it feels more modern and more unisex than [name_m]Daniel[/name_m] and [name_f]Eleanor[/name_f]. The sounds also feel slightly harder than the sibling names.
Lucas- This is a nice match for the timeless, gentle feeling of [name_m]Daniel[/name_m] and [name_f]Eleanor[/name_f] (especially [name_m]Daniel[/name_m]). [name_m]Luke[/name_m] would make a great nickname!
Oscar- This is another name that feels a little posher than some on your list. This name isn’t my personal favorite on the list, but would go well.
Oliver- This seems like another timeless choice. It doesn’t have quite as much gravitas as [name_m]Daniel[/name_m] and [name_f]Eleanor[/name_f], but would make a nice sibling.
Phoenix- I really like this, but it feels slightly edgier and more modern than [name_m]Daniel[/name_m] and [name_f]Eleanor[/name_f]. I think it would make a really great middle name!
Rowan- While I love this name, I think that it doesn’t feel as timeless and [name_m]Daniel[/name_m] and [name_f]Eleanor[/name_f]. This seems like a great name that doesn’t stand out from a great list!
Riley- I love [name_u]Riley[/name_u] for a boy, but it feels different in that it is a unisex name compared to the more binary sibling names. Again, great name, but your whole list is great!
Sebastian- Like [name_m]Henry[/name_m], I think this name matches well in a more posh way. [name_m]Seb[/name_m] is a cute nickname! I think this would make a good choice, but I prefer some other names over it.
Samuel- I love this! It has a very similar feeling to [name_m]Daniel[/name_m] while still being distinct and recognized on its own. It’s familiar but not overused. I think “[name_u]Sam[/name_u]” would go great with your kids! The only concern you might have is pronouncing [name_m]Samuel[/name_m] and [name_m]Daniel[/name_m] in quick succession, as they have similar sounds.
Tobias- I think this is a great choice! It matches the vibes of the siblings’ names, it has a nickname that goes well ([name_u]Toby[/name_u]), and it sounds well with them while still having distinct sounds!
Thomas- On the surface, this seems like a nice match, but it feels a little more stolid than the sibling names. While I like [name_m]Thomas[/name_m], I prefer [name_m]Tobias[/name_m].
My favorites are [name_u]Elias[/name_u], [name_m]Isaac[/name_m], [name_m]Tobias[/name_m], [name_m]Josiah[/name_m], and [name_m]Samuel[/name_m], with [name_m]Henry[/name_m] and [name_m]Lucas[/name_m] as runner ups. It’s true that I tend to go for softer, Biblical names, so my favorites are made up of that even though you have some lovely and fitting names on your list in other styles.
I absolutely love your list and don’t envy you choosing! Maybe a narrowed down list might be easier to choose from once you meet your son.