*Theory Crafting minmax hat goes on*
One of the game design decisions in l5r is how balanced the game tries to make your character. Having one ring lower then the others is a bad idea almost universally, unless you don't want to go up insight ranks. In addition raising skills slowly but surely becomes inefficient compared to raising rings. At skill rank 4, unless you are going for a Mastery Bonus its inefficient to raise your skills higher.
Rank 5 skills are less insight/xp efficient then rank 8 in void, and rank 6 in any other ring.
Of course, Winter Court was a shorter game, and its not like you plan to raise multiple skills to rank 5, so OCCATIONALLY the +1k0 is worth it at rank 6, especially if it scores a mastery bonus.
Most of the time I question if rank 4 is worth it.
Other advice...rank 3 in all rings is just flat out good, and IMO worth more then rank 2 in any skill. A very common TN in game seems to be TN 15, and while 3k2 averages 15.305, 4k3 is a safe 21.910, so you should get more success then failures on both TN15 and TN20 rolls, even with bad luck. 4k2 is only 17.399...
So rings are better both for rolls and at least equal for xp....
So how do you make a specialist character....the Weakness disadvantage is your friend here. Forcefully lowering one of your traits mechanically, but letting you keep the insight, and in fact gaining you xp...if you want your character to be weak in one area and strong in another this is the best way to go.
In a high level game Elemental Blessing makes raising one ring cheaper. But you need to reach Ring level 4 before it pays for itself. Enlightened is the same for Void but requires Void 5 before it pays for itself....
Friend of the Elements gives you a free raise for a particular ring as well.
If you want to be good at a skill, and have very good dice luck, Great potential allows you to make raises for that skill up to your ranks in that skill. But given the cost of raises, and the advantages of having a high void (especially in a combat focused game) to keep you alive, this isn't exactly great.
Finally you have either stuff like Sage, Soul of Artistry, and the like that gives you bonuses to a skill type if you don't have the skill (Lore, Artisan, etc) to show a general knowledge...
Really, being good at X thing requires you to pick either a School or Advantage that improves said thing. With the use of Different Schools Advantage and multiple schools, there is SOME stuff you can do, but both are quite expensive, and unless the GM is kind to you, or you chose very carefully, a lot of techniques are dependent on School Rank...
Which means a Rank 2 Kakita Bushi Rank 2 Hida Bushi, is usually worse then a Rank 4 Kakita Bushi or a Rank 4 Hida Bushi.
The most IMPORTANT exception this is is Monks. Kiho is just great like that....but Monks are rather a special case in their own right.