Time frame sounds fine.
I intend to enter, as it gives me a reason to finish some of my unfinished games.
I think the development time should be unlimited, just as it is for IFComp. If ParserComp is a success and becomes an annual event to supplement the other comps, then you can start on the next year’s entry as soon as the comp is finished.
I don’t mind the registration (or intent to enter), but I personally don’t like to register until I’m damned sure that I’m going to get it finished on time. In game jams on itch.io, you can register right up until the submission time.
Regarding a theme, that could be optional. If the theme is compulsory, then you need a greater lead time to think about and design a game to match the theme. That might put some people off. I’ve had to do that for five Adventuron game jams, so I’m used to it, but I would prefer an open slather for the first ParserComp so that you can gauge interest for future ParserComps. In my case, it means that I’d have to start something new, rather than finish one of those unfinished games that I hinted at above.
One factor that you haven’t mentioned is playing time. IFComp has a judging rule whereby you must judge the game based on the first two hours of playing. You don’t have to finish the game in that time, but you should be able to judge it in that time. As a consequence, I think most IFComp entries tend to be written with the idea that you should be able to complete the game in two hours. Spring Thing is less restrictive on this.