As someone who loves the XCOM series (at least the newer ones, the older ones had just way too many characters to control in one turn making fights way too long) I see it as "I can be lucky/unlucky - but the enemy can be lucky/unlucky too". And you plan ahead according to that. But I see your point about major RNG moments and I really like the 30% chance to get the powerlevel one higher instead of having it do nothing or everything - that makes every point into those stats useful instead of having 2 wasted points until you can invest the third one. Basically a minor RNG instead of a major one. I hope you will have more of those minor RNGs. (like the weapons doing x to y damage, spells doing x to y, etc.) Removing major RNG seems fine to me as it CAN be frustrating. But minor RNG is needed to keep you on your toes no matter how well you plan ahead. Or more accurately: it makes you plan ahead with the worst possible outcome in mind. Or makes you think about the best possible outcome (highest damage from the damage range + a crit) if you are in a dire situation you might lose if you don't try to go for that lucky shot. Having to go for a lucky crit and high damage from the damage range of a weapon or spell means you already did something wrong earlier in the battle but at least gives you a chance to recover from that mistake. RNG is not onesided but going for less major RNG seems fine to me as long as you keep the minor RNG.
And I like your approach of going for 50% chance effects which can be boosted to 100% with the investment of some extra AP with another spell. Maybe even make that buff vanish after doing a % based spell? Basically a "make your next spell/skill stronger and double the chance of all it's effects". That way it would work with all kinds of spells to increase the chance from 50% to 100% but it would only work for ONE spell. Else you could just use all the 50% chance spells together with that buff which would be bad. Could even be a source skill and have stacks, effecting the next 2 spells or sth. I dunno, just ideas.
As long as you still include minor RNG or have spells which remove 50% RNG chances from other spells I don't think anyone will need any major RNG. I like your approach of rather going for 50% chances than 95%. I think having damage ranges for weapons and spells and the 30% / 60% chance to get effects one rank higher as you mentioned depending on your skillpoints invested everyone who likes that bit of RNG will be happy. At least I would be. I mean you still got things like critchance. That way you always have a certain baseline damage with the chance of high damage. Crits, blockchance, evade and some minor damage ranges from x to y are all that's needed to give you those "YES" moments in battles when everything works together. A small question: With a 100% chance to hit - will you still be able to evade and block attacks? If yes that would make it very interesting to use debuffs on enemies AND buffs on yourself to get that 100% chance - making a 100% chance an actual INVESTMENT of AP which would be twosided.