Actually, one of the few fare codes that really makes sense - it's a Oneworld Explorer ticket covering 3 continents that is in first so it books into A inventory.

The fare is what it is. Most people would consider it a much better value than a full fare F ticket.

It depends on the airline. Flights are only loaded 330 days out, so anything beyond that you can't reserve the flight, let alone a seat. If you're trying to cover days that are more than 330 days out, that's not a ticket problem - you just need to tell them the routing and then come back later and select specific flights.

AA - can select seats any time.
CX - doesn't assign seats more than 6 months out
BA - used to have some issues with advance seat assignments for non-full-fare F and J tickets (i.e., A and D) but I think they've changed that now.
QF - shouldn't be a problem but sometimes they won't pre-assign all seats (and you're not flying QF, so you don't care).
AY, JL, LAN - don't know but you're (probably) not flying them anyway.

A trade off.

Book now, lock in the flights you want (and hopefully seat assignments). Get peace of mind. Pay now.

Put it off. Risk lack of availability. Risk price going up. Save 'interest cost' associated with paying now.

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Other things to think about -

The cost of an AONE4/5/6 - covering 4, 5 or 6 continents (adding South America, Africa or Australasia) is incrementally not that much more than an AONE3, so if you have any interest in going anywhere on those continents, it's worth considering.

No matter how many continents you have, you're limited to 16 flight segments (and ground segments, e.g. IST-DXB) count against the 16 limit. You must go from one continent to the next (east or west doesn't matter) but you cannot backtrack from continent to continent. You cannot return to the point of origin until the end. However, there is no requirement to do the whole thing in one trip. You could, for example, go DFW-LHR-IST, then get a flight home to DFW from DXB on a separate ticket (or go to LHR on this ticket and fly home from there), then later on fly back to IST (or DXB or LHR) and carry on with the rest of the ticket.

For example, you could go DFW-LHR-IST-(cruise to)-DXB-LHR. Get a separate return ticket LHR-DFW-LHR to get you home. Go back to LHR (6 months later if you like) and do, for example, LHR-NRT-SIN-HKG-LAX-ORD-HOU-JFK-MIA-JFK-DFW. This uses up only 14 of the 16 allowed segments so you could add another 2 in Asia or in Europe (but if you do them in Europe, you'd have to do them before you go to Asia, as you cannot 'go back' to Europe once you leave).

You're allowed up to 4 segments in each continent and 6 in the U.S. However, you're allowed only 2 stopovers in the U.S. (because you started in the U.S.). It would probably be better to add 2 segments in both Europe and Asia and drop 2 from the U.S. Once you get back in the U.S., don't go home (DFW) until the end so if you want to actually go home in the meantime, go somewhere close (Houston, for example) and drive (or fly on a separate ticket) home. All segments need to be done within 12 months of starting.

Maybe ....

DFW-LHR-IST /cruise/ DXB-LHR //home on separate ticket // HEL-CDG-NRT-SIN-HKG-BKK-HKG-LAX-ORD-HOU-LGA-DFW (with HOU and LGA as stopovers and ORD just a connection). The possibilities are endless ....