Firstly, 3-4 monthly trimming is far too infrequently & this will be a big part of the problem. On average, hooves need trimming 4-6 weekly and when 'correcting' problems, often more frequently is ideal. I appreciate you said they grow slow, but the mechanical stress due to them becoming overlong & imbalanced may also be slowing growth. Also look into diet & nutrition, as they're big factors in hoof health. A 'low carb'(grain, starch, sugar free or low), high fibre diet and well balanced nutrition will go a long way. Of course, it depends what's in your horse's diet/grazing as to what will be needed to balance it, so there's no 'one size' answer.
Your farrier is right in saying the existing cracks aren't fixable & need to grow out. BUT good management is important & it is definitely possible to prevent cracks getting worse & *help* them grow out. I don't believe fillers/sealers are a good idea, especially with hooves that aren't attended regularly. They are a cosmetic measure & don't help fix the problem, but frequently there is infection present in the cracks & putting any kind of hoof goop - filler or otherwise - on them can just make for a better environment for the bugs to thrive in.
I am all for owners educating themselves as much as possible & learning to trim. There are some fantastic sites, DVDs, books etc available too, that you can learn from if you can't do better, BUT it isn't something that you can just follow a 'recipe' approach for & learning about anatomy, hoof function & the principles of balance & such are important, as is hands-on supervised instruction if at all possible. While maintenance trimming of healthy feet is relatively straight forward, 'correcting' problem hooves is a whole nother ballgame that takes a lot of further knowledge & skill. Mayfield Barehoof Care Centre Home Page www.hoofrehab.com(&
Pete's DVD set) are among 2 of the good sites you can learn more from. Oh & if you would like any specific advice & opinions on his hooves, check out link in my signature below & post hoof pics & more info, on diet, environment, etc.