you're gonna need to get up to at least 70ish before you can finish the backyard matches all the way through, but you'll likely hit that while doing other sidequests. what i did was get up to the point where i'm supposed to be heading to grand staff and then start working on finishing up the game. i'd probably recommend doing the mini-boss things early on just because you can gain some good exp just fighting your way to the bosses. at 50ish i'd say probably go to the great ancient ruins place for grinding out levels, or temple of enlightenment if its not too hard at that point (you'll be gaining a level per encounter there up until like lv75-80).
i cant remember what level i was when i did the fight, but you'll pretty much need to get up to being finished with numara story on disc 4 before you can completely finish all the quests (white boa gives you access to several new optional areas). the royal seals thing would be one of the first things i'd finish though: it gives you the best sword you can have for any character in the game (assuming the immortal has royal equipment set)
other than that its pretty much just a bunch of tedious checking over of each area for seeds and items

that can take up a whole lot of time if you dont own a guide, so you could very well be playing for double what you have played up to disc 4
edit: several of the lv3 rings you'll end up using are dropped from the invisible chests or mini-bosses. the only one that i actually made and combined was the assassin's ring. other than that i've only had my main 2 melees using the pair of rings that have hp+mp drain for a cheap way of refreshing and regenning all characters at once :> been sticking mostly with a 3 mage backline for the last disc because alot of it requires more curing than you can cover with just 2 people on 3 non-casters