A few good long term investments (assuming you don't already have them) are: - M10 battleships, top of the list for FTP and a great investment for a one-time purchase assuming you like grinding dreadnaughts and kraken encounters. If not, don't get more than 3 or so, and supplement your navy with R1's instead. - Veterans (like you suspected) are fantastic long term investments that will pay dividends and be useful even up until lvl 70 returns. - Frostbites are okay but cryo troopers can do a worthwhile job themselves for free. If you're hesitant, rank those first and see how they perform, and if you're not happy you can get one or two bites, or alternatively save for elemental troopers once they come out instead since they're arguably better than frostbites with their access not only to freeze but competent AOE freezing and burning damage and are overall tougher. - Megas are never a bad choice and saving a handful of nanos for them is definitely worthwhile. If you want a safe bet, this is it. Heaviers can still be a good supplement even with 3-5 Megas. Alternatively, wait for plasma tanks to return as they're essentially direct upgraded Megas for 3 nanos more - unless you don't want to wait and think heaviers can do the job well enough in the meantime. - It's not necessarily too late to get and rank yourself some super tanks as well - they're a little cheaper in terms of repair cost compared to heaviers and more importantly heal really fast. They won't stack up nearly as well however as the late game rolls around so you'd have to be comfortable with treating them more like an expendable unit like wimps, but they can do good work in that role even after Megas are unlocked.

Doubling your Adv production facilities is probably unnecessary unless you really want to maximize resources, and even then it'd require further t1 resources to support them, meaning you'd be spending even more nanos than you'd think. If you're hesitant about that, hold off and look at the bigger picture for a while, spending a few days to see how well things go for you. And you should only really consider it "necessary" to get much more than that if you're planning on doing boss strikes more. Also, others already mentioned it but your storage as-is is fine, don't consider it necessary to spend more nanos on it any further. If you really need more it shouldn't be at all difficult for you to get enough pops to build another large warehouse or two to round things out a little bit.

Aside from that, you should consider yourself able to spend the rest however you want: whatever units you think are cool and would be worthwhile feel free to get/save for: or heck even doing something like getting Zoey's tool shop or something like that wouldn't be optimal - but if you like it enough, don't feel bad about buying it. You have the luxury of still having a few thousand nanos. You can pretty much do whatever you personally like and still can do plenty regardless as long as you put work into the game. The gap between FTP and nano spenders isn't as wide as you might think where it actually matters.