Anything spicy from being able to cast 6th level spells early?

[Inevitability]

Do you stop qualifiying for your mark of healing if you become a Dragonborn? I suppose that RAW, indeed you do. But you might be able to convince your DM to let you keep it, "it's a property of my soul and not of my body" for example.

I think the argument for keeping your dragonmark is the RAW one, actually.

Member of appropriate dragonmarked race and house

You still count as a member of your original race for the purpose of any effect or prerequisite that depends on race.

[...] the bloodlines of the dragonmarked houses only manifest in the most common subraces. For example, only high elves who are members of the House Thuranni or House Phiarlan bloodlines display the Mark of Shadow, and only hill dwarves from House Kundarak display the Mark of Warding.

Obviously you remain part of your house by undergoing the Rite, so the only question is whether you remain part of the appropriate race. We know 'appropriate race' means something like 'lightfoot halfling specifically' or 'high elf specifically', and you continue to count as such for prerequisites, so why wouldn't you be able to keep the mark?

The default argument against subtype alone not being enough draws on the PGtE quote above, but that doesn't apply here: a dragonborn lightfoot halfling isn't a different subrace than a lightfoot halfling.