Where do you draw the line regarding doping?

Personally, if it falls within the WADA/UCI rules it is fine, otherwise no.

Medical use should always be exempted. People should not have to sit out of races because of a treatable medical ailment. Similarly a doctor employed by a team should still have a code of ethics which means prescribing by need, rather than prescribing for performance enhancement.

In general I'd be against unnecessary medical work, so banning needles and IVs which require much more sterility seems sensible.

I'm not aware of anything other than caffeine and EPO having statistically significant performance enhancing effects, although the lack of studies may have something to do with this.

Finally there is an argument that a properly funded UCI acknowledged doping programme might be the answer. EPO on a regulated programme is a lot cheaper than altitude training camps, seeking to get the same benefit.

If it is sanctioned and authorised, then no-one can be accused of cheating. Riders can have a confidential list of substances they are using, which there is no point lying on because there is no punishment, and the effects can be monitored. Studies can then show which drugs actually work, and which don't.
New drugs can be properly safety tested, rather than being used immediately because they will soon be banned and tests for them developed.

Dietary supplements, vitamins and prescribed medication for long term health problems such as asthma.

Anything on top of that, which you need to see a doctor about and have something prescribed. Firstly it should be a UCI contracted doctor, not a team doctor. You get what you need, you can have ONE Therapeutic Use Exemption (TUE) for that medication, but if the problem persists, you sit out races until you are clear of whatever you needed to take to get better.

I could go into detail about other aspects, but the biggest on is the use of team doctors to prescribe things and TUE's.