is it possible to be able to buy cryptocurrencies other than bitcoin, litecoin and eth with USD
Your issue makes no sense; the price of the intermediary currency is irrelevant.

Say you want to buy $600 worth of Bloopcoin. Bloopcoin's exchange rate is around $60 per Bloop, so you expect to get 10 Bloop.

Unfortunately only markets to buy with BTC are available, and (assume) BTC's exchange rate is $6000 per BTC. You can still buy 0.1 BTC for $600, and then use those 0.1 BTC to buy 10 Bloop.

If BTC were instead to be at $12000 per BTC, you'd only be able to buy 0.05 BTC for $600; but if the Bloop exchange was $60 per Bloop, those 0.05 BTC would still buy you 10 Bloop.

I'm aware this is not a full answer to your question, but perhaps it addresses the "since price is so high" comment you made.