I don't believe you lose the OP warps anymore changing nations but even then, there are other ways to port now including telepoints and what they call survival guides (looks like Fields of Valor books). Touch them once and you can port there from wherever there is another telepoint/guide for tabs/gil/etc. The maps now include icons to where they are located for each zone. Not sure if you have to be in the zone to see them though.
Maybe look into picking up some alter egos to help smooth some things for some of the quest/mission fights you plan on doing though certain jobs could probably solo without much difficulty since the level cap on most if not all have been lifted. Being a duo team works well with most things in the game anymore.
To get the avatars (at least the basic ones) you should be able to do the 75 fights relatively easy as 99 War. Maybe bring a vile elixir for the two hour (called one hour now?) move but it shouldn't be that bad. Again, alter egos could help with this as well whether you do them solo or duo.
Start the Spark quest/objectives so you can get some basic high level gear for 99. If really needed while leveling you can get gear for those levels too but the choices weren't to great last I looked. Other benefits to the sparks objectives as well such as XP.
Things are much more lax and easier to get through till end game now.
End game related tips, I will defer to whoever wants to add to it.