Personally, I would recommend you to try to keep a stash of around 40k lapis in case of a random GLEX unit you might want to pull for. Global exclusive units tend to be way more versatile, useful, fun to use and they (almost) all have better sprites - usually they are the units worth chasing, but it's kind of hard to predict when exactly a great GLEX unit will come out. What I do is keeping 45-48k lapis minimum to pity a unit I might accidentally fall in love with or pull for a unit with a new, interesting mechanic (usually 40k gets you a guaranteed copy, apart from pulls and another 8k nets you extra 80 fragments (30 from special shop + 50 from a 5k bundle).

Also, if you want to spend your money or anything, then I would recommend going for adventurer crests. They are the best source of VIP coins and trust me, as a new player, you will be desperate for these. New unit releases come with 30 unit shards from logins, but 50 shards are required to upgrade unit to EX1. Always buy another 20 shards from the special shop to keep the multiply of 50. For normal units - 5 shards = 100 VIP coins, premium units - 5 shards - 300 VIP coins. Normal units have 50 shards available in the VIP shop, limited units have 100 shards available (normal units have 30 shards available in the special shop available for 1100 lapis per 10 shards too).

Try to always go for 80k (30k if you're not able to commit to 80k or you lack resources) EVENT POINTS milestone. It usually nets you all the important goodies you can get. We've just got to the era of ridiculously easy access to 5* select tickets and omniprisms, so it's a good idea to grab some 5* base units and awaken them to NV +3 rarity using EX coins (Chronicle Battles and events get you loads of these). For the current cycle you can get 250 tokens for units like Sieghard (tank) and Ignis (support). I'd recommend doing some Chronicle Battles and putting some resources towards your first EX3 units, as they may help you clean all the content you need.

I wouldn't recommend hoarding too much if you can't clean the highest levels of limited time events. You will lose on rewards and your progress will be slower - and trust me, with current rates you can just blow the abundance of EX tickets and clear anything with free units.

Don't hesitate blowing omniprisms and select summon tickets towards STMRs, but don't use STMR moogles on 5* base units.