I'm just not sure if I should buy the MG Chem 835
This is what I use. I do a lot of soldering, and anything that is $10.00 a syringe would be significant expense. I can use 6-9mL of RA on a given day. If the flux is a significant expense to you, you will choose your flux like any other high quantity component... the way exe does. You'll find the cheapest stuff that works for what you are doing.
A lot of people are into gel flux, lately. Amtech 599, along with many/most other gel fluxes, are/were originally designed for BGA. The gel prevents the solvent from drying out as fast, so the flux will remain active for a longer duration during preheat and ramp up with hot air. Probably one reason why someone like Louis Rossman uses it on everything is because he works on multilayer leadfree boards using tiny iron tips at very high temps. This kind of flux can take more heat without burning and building up residue on the iron tip. But he cleans the boards, after, in an ultrasonic cleaner. If you don't want to clean the board, you generally ought to prefer a flux that dries out faster.
For lower temp work, 2 layer boards, and lead solder, you actually won't realize as much of the benefits of a gel flux. But OTOH, if you are like most hobbyists, a syringe might last you over a year, so the cost isn't a factor.
I wouldn't buy the 835, yet. The two syringes you bought are flux, and flux is flux. But if you find a need for larger quantity, the 835 is $25.00 a liter.