Hi ClareJ
For that routing, most trains won’t accept reservations so seating for four might not be guaranteed. For the Basel-Strasbourg TER you should be able to get seats for four as Basel is a terminus, but the seats for four are in the middle of the carriage. There might be one or two TGVs from Basel to Strasbourg that would require a reservation, but that will be a much more expensive and restrictive option over the half-hourly TER.
Also, how big are your bags - can you measure them? Swiss trains will have overhead luggage storage to fit medium-size luggage that would fit in the overhead on a regional or smaller mainline aircraft (e.g., Embrarer 175/190, Canadair CRJ 700/900, smaller Boeing 737s or Airbus A319/320), but I would not expect a lot of luggage storage at the end of the car - especially as you won’t know what type of train is serving your route until it shows up at the station. See this SBB link for more details.
The Basel-Strasbourg TER does not have any larger luggage storage at the end of the cars from what I remember.
I don’t know your situation, but you may want to consider packing less and expect to do more laundry. Also, if you can, invest in AirTags or similar devices - they’re great for keeping track of luggage if you’re worried about it walking off, or forgetting about it and leaving a train without it as I once did!