Par meters are expensive so thats why i suggested borrowing one, you can ask your lfs. If that doesnt work out you can use a cheap lux meter or i believe good phones even have an app you can use worst case.
@saltyfilmfolks can fill you in. I have a par meter so i have never messed around with lux. It can be very hard to tell how much light you are giving your corals just by looking so a lux or par meter will help you make sure you set up your leds safely. The alternative is if you have a fixture that other people use succesfully, ask them what their settings and mounting height from corals are and try to replicate that. When in doubt, less is better.
Your tank is done cycling so you dont need a nitrite or ammonia test kit imo. It would not be a bad idea to have a nitrate test kit, i recommend red sea pro. I would suggest you buy a nicer test kit for dKH and calcium like salifert, but if you cant afford it then your api should work. Salifert test kits are very reputable, accurate, and cheap. There is no need to test pH as it changes throughout the day and day to day, plus you shouldnt try to control it, especially as a beginner. It will normally stay in an acceptable range. You do not need a strontium test kit, especially as a beginner. monitoring it is certainly not essential and you definitely dont need to just blindly dose strontium. Unfortunately your LFS owner doesnt quite know what hes talking about or he is just trying to sell you things.
I recommend you test alkalinity/calcium daily at the same time as you learn more about your tank and begin to dial in your 2 part dosing. Then once it is in check you can test less frequently if you like (i recommend weekly at a minimum), but most people who keep sps continue to test alkalinity daily. I recommend you always test alkalinity daily as you dial in your dosing/need to adjust it from time to time. This will prevent you from getting large dKH spikes like you had recently. magnesium shouldnt move too much once its in the right range so you can test it every 2 weeks or even monthly. once you start dosing you can calculate how much magnesium you need to add based on how much 2 part you are adding as well.
Does your lfs tell you that he is giving you 1.023 water? there is no reason to run your reef at 1.023, the typical range is 1.024-1.026. swing arm hydrometers are notoriously inaccurate as you said so again do yourself a favor and buy that refractometer (i recommend bulkreefsupply.com if your lfs doesnt carry them). its possible hes giving you 1.024-1.026 and you are just testing it inaccurately. Also, i know your on a tight budget but you will save money in the long run by making your own rodi water and saltwater. You can also pick your salt mix based on what parameters you want your tank to run out, that way when you do water changes there wont be swings. In the meantime ask your lfs owner what salt mix he uses so you can look up the parameters of the water you are getting. Just something to think about.
If you find your salinity is actually at 1.023 you can either leave it there or raise it to 1.024-1.026 slowly. Do you have an auto top off or do you manually top off the tank? Either way thats essential to keep the salinity stable after you dial it in.
Depending on how many corals you have in your tank you may not even need to dose two part and weekly water changes would be sufficient to keep your parameters in check, you wont know unless you test consistently. You should never dose something unless you need to (because your test kits tell you so), it only complicates things and the goal is to keep everything as simple as possible and the tank as stable as possible.
If this all sounds like too much then you should consider sticking to easier corals like softies/lps until you are more experienced and then you can graduate to sps. They are more demanding and usually require more testing etc.
I strongly recommend you watch the following series on youtube. This will cover everything you need to know to run your tank succesfully. its definitely outdated but a good place to start. The second link is a much longer more in depth series. There is plenty you dont need to know in those but they do dive deeper in some of the topics you should be familiar with. it is also more recent/polished. Id watch the first series and then move onto the second.
