I have been wondering for a long time why couldn't you have a space capsule that is lifted by multiple hydrogen-filled balloons, and when it reaches the maximum altitude, it would pump the hydrogen out of the ballons and use it for fuel to continue its journey upwards? At the maximum balloon height, air resistance would be minimal. The temperatures would be cold making it easier to store gasses under compression if you need to.
I assume that ordinary balloons will break or be fragile. Perhaps tougher materials can be devised for this.
You will also need oxygen. The only question is if you can get enough from the atmosphere when you need it, or if you have to store it. I'm thinking that maybe you could work out a process to acquire and store it as you rise.
To get a better technology, we need a really dense energy source, such as nuclear power, nuclear fusion, or antimatter. The last two are beyond our capability, for now.