You've done a much better job taking advantage of what you've been given in this isolated start compared to the Mehmed game. First in GNP and production is a strong position at this point.
I don't like the idea of getting Astronomy via Liberalism's free tech. Beelining Optics was the right move and you've done a good job getting there so quickly. But while it will provide you with the opportunity to trade techs with other civs it doesn't solve your major problems associated with being isolated. You still need Astronomy to:
1. Get foreign trade routes.
2. Trade resources with other civs.
3. Have religions spread to your cities.
4. Use galleons to send settlers to claim any unsettled landmasses you find nearby.
5. Use galleons to send berserkers to liberate any nearby landmasses that have been settled by AI civs.
For these reasons I think you need to make getting Astronomy a higher priority than waiting for Liberalism. Once you've finished optics you can get to the point of researching Astronomy quickly. You'll only need Math->Calendar.
While Astronomy is an expensive tech it should be possible to use a GS to bulb about half of it, and possibly even use 2 GSs to bulb it entirely. According to the GS tech list kniteowl posted a couple of pages back the only other tech you would need to get out of the way for the Astronomy bulb would be Alphabet. You would need to avoid Meditation, CS and Theology to block Philosophy and Paper.
If you build NE in Nidaros ASAP it will produce 2 GPs in the next 35 or so turns if you continue to work the 2 scientists. Both will be highly likely to be GSs (about 70% for the first and 75% for the second). That's about a 50% chance of having both be GSs, which would mean getting Astronomy around 600 AD.

Even if only one is a GS you can still research the remaining part of Astronomy in about 20 turns which gives you Astronomy around 800-900 AD, which is much earlier than you can get there via Liberalism.
You would then have a monopoly on Astronomy for a long time. It seems appropriate that the Vikings should be the only civ with the ability to travel the high seas for centuries. And if one of the goals of the ALC games is to play each according to the leaders strengths it seems this is the perfect opportunity to prioritize Astronomy. How many times have you done the Liberalism beeline? (Actually you can still go for Liberalism after getting Astronomy in the near term.)