Any collection with a marathon and a tuna can is a good collection in my eyes. For you as odd as this sounds I feel like you need a modern basic watch to round it out. Gs snowflake is my first thought. I also think given what you have you would appreciate a mm300.
I dont know if I would say it is my “grail” piece in the traditional sense but when the sbge275 was released It really hit for my tastes and wanted to see it in person. Ofc they sold out before I had a chance to look at them. I went back/forth for a year on buying one second hand until I ended up getting it from my wife as a wedding gift. Thats probably the most grail piece I will ever own, has value to me far beyond just being a watch.
I have a casio 5000 that I payed $30 for and I put it in for My worst of days when I am doing things that are very likely to damage a watch. 3 years later it is still in perfect condition other than a few scratches on the plastic from things. I am pretty sure my wrist would break before that watch does.
Depends on the style you want obviously as there are a bunch of good options at that price point especially if you go used but my personal choices would be sinn or a seiko mm300