1. I do not recommend any OME rear coil with bilstein 5100. OME rear coils are way too heavily sprung for bilstein 5100 (imo for stock weight 4runner as well). I've run 2895E (shorter coil compared to 2895, same spring rate) and they're like concrete. They do handle wonderful on road, that's about it. Plus, bilstein 5100s are tuned for stock spring rate.

2. A better way to get more lift in the rear is using spacers or other lift coils with less than 220lb/in spring rate (Dobinsons, Eibach, Bilstein). This way your 5100s wouldn't get overwhelmed with crazy stiff springs. If you are doing a rear bumper or ROUTINELY carrys a lot of weight in rear, then go for heavier springs. Also, spacers wouldn't be a problem in the rear since your rear droop is limited by shock length not spring length.

3. Best way to get more lift in front is by cranking the preload up to the top notch (also not recommend: with FJ coils and 5100s at 1.75 you're already looking at around 2 inches of lift; at 2.5 inch perch you're looking at 3+ inches, which is a lot of lift). Those shocks are tuned for stock spring rate, and they've tested them with all 4 notches, so you can safely use all the heights. You can get OME coils for marginally better handling, but not necessary just to get more lift, and they will not solve your wandering problem.

4. Like other replies mentioned, aftermarket high caster UCAs don't need to adjustable, and if aligned properly they can make it drive better with better steering feel and weight. If your other components are good, LCA bushings, tie rods, they may even solve your wandering problem. Mine's steering has a bit of play and is a bit stiff, but using aftermarket UCAs running almost 5 degrees of caster made it wander a lot less on highway. You can play with a bit of toe-in as well, setting it to the factory limit.