The main concern I had was that it leads to a development of polyethylene glycol resistence. After talking this through with a doctor, I now believe that 1-2 doses of the vaccine are unlikely to have a significant effect but still some that giving significantly more doses (for example 2 every year to target changing viruses) could lead to such an effect. 

I already fully isolate, and would only value my freedom a bit, whereas I value my health a lot.

Human contact is very important for human health. The idea that you can fully isolate without paying a price for it in health seems questionable to me. Humans are not constructed to live healthy lifes in isolation from other humans.

Epistemic status: someone told me that someone that knew about this told them that

mRNA vaccines have been done on non-human animals for decades, and they seem fine

The mRNA tricks your cells into making a spike protein

The alternative is the live adenovirus vector, which tricks your cells into making a spike protein PLUS all the other proteins that make up the adenovirus.

So it seems like the former probably can't be any worse unless it "infects" different types of cells.

The new lipid nanoparticle delivery method can potentially target a much wider range of cells than older modalities. Any cell can ingest an LPN and start displaying spike protein epitopes. All the cells that are not professional presenter cells and do not have MHC-2 complexes will be destroyed by immune system. Some critical, sensitive, hard to replace cells might be so killed at random if enough LPN get into systemic circulation which can occur if for example a needle pierces a blood vessel during vaccination and a large portion of LPN spills into the veins. The chance of this type of accident and the chance that significant damage to your organs will be done as a result is not very big, but it is a risk which you will be taking every time you vaccinate.

I'm not a biologist, and bodies are complicated enough that I'd like an answer to this too.

But to a first glance from a layman: I have a hard time understanding what mechanisms there might be, especially age-dependent ones. The sugars, salts, acids, and stabilizers all either occur naturally in the body or have been used medically long enough to have their risk profiles known. I don't know about the specific lipids used, can't comment there. mRNA only survives in the body for a matter of hours, after that what's present is just the proteins you made from the mRNA, which should work the same way as identical viral-derived proteins would, generating an immune response by the same mechanism.

Instead of an either/or decision based on first principles, you might frame this as a "when" decision based on evidence.  We've had about 4 months of real-world experience with the mRNA vaccines... if you wait another 4 months, that's double the track record, and it's always possible that new options will open up (say, a more traditional vaccine that's more effective than J&J).