If you want to plan your trip in a fair amount of detail and see the various possible destinations and alternative routes there is the European Rail Timetable, which is (I think) available in both book and digital formats. You can use it to work out your out and back trips, look at alternative routings, possible short day trips, etc. It is in English, so no worries about staring at departure boards wondering what that word means. (Or getting to a destination on a Saturday and finding that the train you planned to return on is Not Saturdays.) If you do get it, it is best to spend an hour or two familiarising yourself with its layout and symbols. It doesn’t cover every line in Europe, but it is amazing how much it does cover.
.europeanrailtimetable.eu
Yes - my starting point when planning rail travel in Europe (unless it's a very straightforward point-to-point journey I've done before) is generally a combination of the latest ERT plus relevant tips from Seat61.
There's no one best place for getting the tickets, given the lack of integration of services and the competitive nature of rail companies these days. But if you don't have much experience, then just follow Seat61's advice, which includes suggestion for best places for ticketing for various places. If you're just getting a Eurostar ticket, plus odd local ones at your destination, then of course the Eurostar tickets is to be got from them, and the other odds and ends when you're there.
If you're not leaving until the Friday evening, then you're pretty much stuck with only reaching Paris or Brussels, or somewhere quite close to them, that night, and then having to get to anywhere else you want to actually be on the Saturday morning. Which cuts out some of your weekend. (Also, as has been said, there's also Harwich-Hook on the overnight boat, which means you could get to many places in the Netherlands easily during the Saturday morning.)
You mentioned the Wuppertal Suspension Railway - which is indeed an interesting experience. I was thinking if you went to Brussels, and then hopped over to the German border, say to the first place (Aachen), on the Friday night, you could explore that region of Germany (including Wuppertal) all day on Saturday. The problem is that the evening London-Brussels train on the Friday would miss the last connection to Aachen (either on the intercity, or on the local cross-border connections), so you'd either have to go only part of the way, and stay in a smaller place in Belgium towards the German border, or you'd have to leave London in the afternoon so you could get into Germany on the Friday evening. (Aachen isn't much more than an hour from Brussels on the faster trains.)
English is pretty widely spoken in main places in that part of Germany; Aachen itself is a nice smaller city; and you have the opportunity to hop over into the adjacent corner of the Netherlands too. If you were spending a couple of days based there, then there are regional travel-where-you-want transport tickets which are good value. (And of course, right now, the are Euro9 tickets for the whole of Germany lasting until the end of August. But I guess the trip isn't that soon!) Note that if you're looking to travel inexpensively, and want to use a Eurostar train on a Friday, you need to book a long way ahead!