You can use a temporary 3M window film that goes over the whole window while you are looking for longer term solutions. They run about $5 per window at big box stores or amazon.

I'm sure that this is already known, but your windows are are as old as they are poor performing. hopefully replacement is on the radar for the future.

The 3M film recommended above would do the best job of sealing those up for the winter. Seal and peel type caulking may help on specific problem areas, or stuffing soft foam in there can help (sounds like you already have). You won't have much like with any sort of rigid foam in this application.

A living example of why, paying less...ultimately costs more.

I realize that you didn't buy them, just illustrating the poor purchasing decisions of the previous owner is all.

The windows are only 6 years old but there is no manufacturer name on them. I've used the 3M tape but looking for long-term solutions since we do open these windows in the spring/summer. I really think there is some material out there which is more rigid than soft foam that could block out this draft - could synthetic steel wool do the job or something that expands a tiny bit? Or should I try using great stuff sparingly and cut out the excess?

Put the film over the windows in the winter and then take it off in the summer

1) You have at least two types of windows. The first pic shows a window closer to 26 or 36 years old than 6. Notice the aluminum boxed spacer and mechanical fastened sashes, both features that have been obsolete for years. These are defintely due for replacement. The second pic looks like something a little newer off of a box store shelf. Unless the previous owners were heavy smokers, the yellowed tilt latches suggest that they are still likely older than 6 years, but I suppose that its possible. Either way, you don't have a long term solution as far as any of these windows are concerned.

2) Not sure what you mean by "tape", but I'm referring to the plastic that is applied over the entire opening seasonally. Its a PITB and ugly, but its effective. If you simply don't want to use that, the other option that I mentioned would be better than any foam. "Seal and peel" caulk will fill the voids and then harden to seal the area, but unlike regular caulking its designed to peel off in the spring. It may be a little tricky to remove from the balance cavity, but not impossible. Great stuff will permanently seal your windows closed and stain them. Steel wool might be able to scrub off some of the yellowing on the plastic, but it won't stop any air leakage.

A number of years ago I recommended Indow window inserts to a client who lived in historic district and couldn't change out their leaky, poor performing single-pane wood windows. They're worth a look:

https://indowwindows.com/

guys - 2 good ideas - seal & peel caulk and the window inserts. Let me research both. I can't justify the costs in replacing 35 windows for the amount I'll save in this 100 year uninsulated home

Indos are fine and definitely an option for a historic window, but for most folks with non historic options, they are overpriced for what they are. Last time I looked at them, they nearly approached the cost of a new window, which is saying something given the lack of materials and complexity of the product. A single sheet of poly-carbonate (or similar) and some bulb gasket material?

+1. There is a place for the interior "storm" inserts like Indo, but you may be surprised by the cost.

Two other things to factor:

1) Degradation of the existing units will result in seal failures, warpage, broken parts, etc as time goes on.

2) Storms of any type (interior or exterior) can be problematic on vinyl windows as the air space between the storm and window can get warm enough to deform the vinyl under some conditions.

I'm not saying that replacing your windows is the only solution here, but it is for the long term. I'd really try to minimize the amount that you spend to "help" these out in the meantime, whether that be a year, 3 years, or whatever. Your windows have about a 10 year service life.

You could put up some honeycomb shades during the interim until you get new windows. Obviously it's not permanent solution, but it will help insulate the window.