So Directly Put everything in tank and do 50% water change.....
Yes, you can modify it later to suit your habits and routines, light intensity etc.
Commercial brands are water.............with a little tiny bit of fertilizer in them, you pay for the bottle, the marketing, the company's profit/overhead, shipping the weight of water etc......
Since you do good size water changes(does not take much longer to do a 20% vs 50%), there's no need for testing.
You can slowly reduce the amount of ferts added, then reduce the water changes also if you so chose, but some eyeballing is required and how little you can get away with before the plants suffer etc takes some trial and error. Less light makes it easier overall.
Excel, when used correctly according to the label, should never kill anyone's plants or fish, or shrimp. They likely died from some other cause(many ways that aquarist even without plants and ferts, they kill fish all the time for unknown suspected reasons).
I have used Excel a fair amoutn on really sensitive fish without any issues for years. Angels bred, Discus bred etc.
Regards,
Tom Barr