Yes for BB, no for subs but people differ. I hate subs and fins them useless, other people use them a lot. But I do win most of my games so I think my tactics work.

I research all ships and lvl 1 sub(but I don't use the sub). I usually have 3 destroyers per BB but I have more if I can. I try to have as many cruisers in a flotilla as BB and I make a small force of destroyers and cruisers to ferry troops when the AI build subs.

Nuke BB is a game changer for people who are surrounded by a lot of water. In my opinion, the only countries that thoroughly benefit from researching Nuke BB are the Americas (both US and Canada), Sweden (if he survived long enough from Finland or Russia -22 map), UK (if he survived from any country from mainland Europe), Northern African countries, especially Egypt and Morocco. Its a game changer since it acts an extremely powerful defense buffer on the coastline. This is assumed that there is another country that is much stronger than you. Nuke BB can make his attack on your smaller nation extremely costly, even if he has other ships. If it assumed that he has been improving to research mostly ground and air units and only mediocre navy, you could definitely focus all your research time reaching nuke BB and this would get you on his level as he cant touch you. Use that delayed time in his naval power to build up a ground army. Don't neglect your naval superiority and keep your Nuke BBs in tip top shape.

Nuke sub is still good (my opinion), but not as useful as nuke BB. It is an excellent ship escort. I had a talk with Buter about this before, and I mentioned that since it has a ship class armor compared to a regular lvl 6 sub which has a submarine armor class, its an excellent destroyer killer. This protects even your regular subs as well from those pesky destroyers. If anything, nuke sub's value on defense and their use for offensive purpose (except attacking destroyers), is questionable.

The ABB isn't really a game changer, a lvl 6 can do its job(albeit slower) but I stack all types of ships together so they work very well all together. It seems that the lvl 6 is more efficient than the ABB.

That is weird though, I had both BBs in on one game where the regular BB dispatch my opponent's ship faster than ABB. I suppose it is true, but I just feel that the use of ABB is still highly valued since that you still need to use oil for ground and air units. I have more ABB than regular BBs in my navy. Cant really afford to use up a lot of oil by navy alone and still need to expand for the win.

I have 400+ spare oil and 600+ spare RM every hour that I could build stuff with. I am not though, I have a fleet of nearly 50 ships. Might grow it though.

Its just convenient for me. Could use that oil to produce more MT and M inf. I always try to balance my ground,air and naval power and ABB help me create that balance without putting too much strain on my oil consumption since the other two (ground and air) are oil-hungry units.

I always think a strong navy and ground force is key. I find that air forces are not as needed but can still be used with devastating effects.

Air is very good, though don't use strategic bombers, tactical bombers are very useful taking down those pesky incoming ground units and it forces my opponent to make anti-air units and interceptors and waste resources which can be consumed by my massive MT and M inf force along with artillery support from the back. I usually have enough interceptors to take out any upcoming enemy interceptors (abit lower level than them since I focused on ground units tech and naval units for the time being). Since in some games I have, people didn't focus too much on air, I didn't have to research too much on air higher than level 3 interceptor and it more than enough to do the job with a levl 4 tact bomber. if I focused too much on air, I'm neglecting my ground and naval tech. I don't go for cruisers so that save me a lot of tech time to do it on air instead and I don't do HT either, same more time for air.

Try turning off the barracks, they consume a lot. I find they are pretty cheap when you get to a position where you can use them.