The whole "altcoin" thing is just a big clusterfuck. I wish I NEVER touched any altcoins. It just makes your taxes basically IMPOSSIBLE to fill after you get involved. The damn coins die so you cannot access your wallet.dat anymore as the blockchains don't sync, the exchanges die and you lose trading history (in case that you didn't save it which is most likely the case). It's just a big fuck up. Im getting rid of any alts I still have, or maybe not even that, I will just let them rot, they have only given me headaches. If you try to convert any BTC to fiat, you are going to see a nightmare of impossible-to-fill taxes.
Or just don't report any taxes in this specific case. Seriously, if you try to do the good thing but don't have any documents, screenshots, spreadsheets, etc, you're basically fucked.
The government doesn't care about how much of a brave citizen you try to be by reporting your taxes in all honesty, they are out to extort you till they have what they (very wrongly) think you owe them. My motto is; what the government doesn't know can't hurt you, financially or physically. We are a bunch of criminals and tax evaders from their perspective, so we can only do wrong.
As for the altcoins you still have, wait for the next bull run and convert them to BTC, that's the smartest thing to do. I do that too with the wide variety of shit forks that I still haven't claimed. Even the worst possible shit coins pump hard against their BTC pair, so there isn't much to lose here.
Ehh depends on the country, personally in Spain I reported my gains in crypto last year and they were quite ''generous'' to overlook a lot of transactions and other evidence that I didn't have. In fact I almost only needed to provide the exchanges trades and a few screenshots here and there since bitcoin is still not fully regulated here. Problem is that if you keep evading taxes they will bite you in the ass one day, specially if you are planning to make a substantial amount of money through cryptos, you will eventually want to buy a house a car, etc, how are you going to do it?
There are only two things unavoidable in life... death and taxes. xD