maybe cut two temporary wood braces for after removing the permanent ones they will be there for support in spot thats not in way and then quickly pull sump and throw back in the permanent ones… also if you can drain as much water as possible probably safest… id unscrew them and if theres still a ton of pressure on them youd know better what your dealing with…if they slide out then most the weight on four corners…redsea peninsula has a brace that goes in after sump is in but id probably feel safest draining as much water as i can when i plan on replacing sump in near future…
Those 2 front braces, which the doors close onto? looks like 1 x 2 inch lumber, plus you forgot the center piece which 2 doors are hinged onto. That's 3 in total.
I'm assuming the tank is supported by a 3/4" plywood wall, which runs continuously along the sides and back., there may also be a 2x4 legs at each corner? I can't see.
If so, I do not believe the 3 braces on the front are load bearing. They are just holding the doors and preventing center sag. (Imagine the stand if it was open cabinet, do you think it would hold?)
(There should also be cross beams going front to back, underneath the floor of the aquarium, or the roof of the stand.)
As mentioned above, using 2 temporary 2x4 braces, at each end, plus a 2x4 beam, tightly fitted above the braces going across entire opening would reinforce structure indefinitely, while you remove the sump. This would form a Bridge.
Would I do this? no, I'd empty it out. move water and livestock to temporary Home depot 22 gallon black storage bins, (the ones with the yellow cover). Because after your all done, you still have to install your braces and doors back again, using new lumber.
As for the simplest solution, repair the sump in place, clean it out, dry it, reseal it. You don't be able to get the silicone in between the glass panes for maximum strength, but if you do a good job, it will last. Needs to be super clean, remove all exposed silicone, (since the sealant wont stick to old silicone, you would need to reseal the entire sump, along all 8 corners, with plenty of contact with the glass. Not just a 1/8" bead, but more like 1/2"+. If the glass isn't clean, the seal won't hold.
You should find a carpenter to examine the stand before trying anything, for a professional assessment.