If you are going with an 'early in space' theme then I have a possiblity that no one has mentioned (unless I missed it). It is the idea I used once in a space campaign. Basically, it is the use of Warp Ferries. The tech to go faster than light is still new and takes a lot of energy/machinery/whatever. Warp capable ships are thus HUGE. No small craft can manage it. The ferries are like small cities or space ports in themselves. But . . . the ferries have to stick to open spaces. They can't go into systems or too near stars or other collision hazards. Thus PCs would then have to hitch a ride on a ferry to go places, then sublight it a little when they get close. The travel time is determined by four factors, how long before the next ferry arrives, how many ferry transfers they need to do, how fast the ferries travel, and how close they get to their destination.

This has a few of advantages.

1) No need to overthink the problem. The PCs travel time is determined by the speed of plot. You need them to take a long time, then the next ferry isn't comming for a while and it is a slow beast. On the otherhand if you want them to get somewhere quick, they can hop a ferry right away and the journey is a quick one.

2) You can use the ferries themselves as adventure locals. A giant techo-city hurtling through space at faster than light . . fun fun fun.

3) You can impose long trips if you need. These would be timeframes remaniscient of the age of sailing ships. Say the PCs are exploring . . . they take the ferry to the latest new route destination then head out to explore at sublight. Taking weeks or months to get to someplace new. Perhaps they will find a safe destination worthy of a new ferry route. This also allows for rewardable suffering. The PCs are the exporers who do the hard long trips to scout out new ferry destinations. Its tough work but they get the recognition when they do things like find an earth like planet (to use your example).

I know this isn't exactly what you were asking. I guess what I'm saying is that, IMHO, you shouldn't make a system for speeds and such. After all you are intending a less rules intensive play. As soon as you make a chart or speed table or any of that, you have a rule set that constrains you. Just pick a time unit that suits your vision, whether it be days, weeks, or months and use it as you need it. Personally, I would favour the longer end of things but that is just me.

Seriously, there is no need for some precise star chart with defined measurements and detailed speed ratios. As an added bonus you can map out your star chart anyway that suits your fancy without having to worry about your PCs grabbing their rulers and calculators and out mathing you.