You are going to be getting a lot of suggestions for removing some or all of the DoS elements you are introducing to the 5e rules universe. If you are adamant about sticking to this, please, please, I beg you....at least provide a difficulty level or selectable option that removes those additions and forces closer compliance with the rules of 5e. For example, the 'true' 5e rules should have Firebolt doing a d10 damage with no burning effect on the surface. Just one of the many examples that would be impacted.
1. As mentioned, please stick closer to the rules. Stop having every surface area be covered by Acid, Fire, Brine, whatever. There are very specific rules for what can create a 'surface' effect in 5e, most involving save DCs and specific expenditure of resources (leveled spells, class features, etc). Absolute adhesion to the rules is impossible, but right now the amount and usage of surfaces makes the game DoS: D&D DLC rather than an actual 5e successor to the BG series.
2. Please tighten up the underlying grid you have. At the moment, the 'area' for area of effects spells shows the underlying grid system and it shows why it makes combat feel so janky. They are so small that you even have tenth of a feet measured out. That shouldn't exist in 5e and it makes it extremely difficult to judge whether you might be in an AoE or not. The 'area' for AoE effects should be static. You can see great examples of this in Wasteland 3, PKM, etc.
3. Please give a better movement indicator. I often can't even see the 'line' because of all the surface effects or camera position. More importantly, I can't tell whether I'll be in range for a standard attack or not.
4. Rename "backstab" to "Flanking". Because that's what it is. Directional Facing doesn't exist in 5e, and it is clearly another port of a DoS effect. Make it so "Flanking" doesn't require you to be 'behind' your target, just on the roughly opposite side of them from another friendly character. With how janky the grid movement system is, it is impossible to tell whether you'll be in the 'backstab' position because you didn't port over the DoS backstab indicator.
5. Change how the rolling system works. Please, PLEASE change this. I don't want to see the number I have to roll on a D20. I want to see the DC and my modifiers. Otherwise, as you level 'up' the numbers you see on screen go 'down'. D&D tried this in previous editions (for example. the THAC0 system) and people hated it.
6. Tied to my first statement, but change all of the 'extra' combat options you added (Jump, Shove, Dip, etc.) to require you to use your Action, rather than Bonus Action. Having them be Bonus Actions right now severely diminishes the value of class features like a Rogue's Cunning Action, a Monk's Ki, spells like Misty Step, etc. Having a spellcaster Disengage from an enemy that has ran into melee with them should be a big sacrifice for the magic user, either in using their Action to Disengage, or by using a leveled spell.
7. Add the Dodge action. I don't know why this isn't in the game already. It is a major element to certain classes as well as Combat in 5e.
8. Allow for more than 1 'follower' per character. Right now, if a Ranger takes the Find Familiar Option, and then later selects Beast Master, they've just made their Find Familiar option completely useless because they can't use both at the same time. I've already had the game bug out and have both on screen so the engine can handle it.
9. Add a button for switching between 1 and 2 hands for a Versatile weapon. For example, I might want my Ranger to wield his Longsword in 1 hand to take advantage of his Dueling Fighting Style. At the moment that's impossible without forcing him to equip a Shield.
10. Please do another pass on your spell effects. Remove the screen 'shake' for when you select a spell but before you target it. Again, I know this particular effect is a DoS port and you like the effect, but it makes it really hard to argue that the game is an actual spiritual successor to the BG series. For example,
https://www.youtube.com/watch?v=Zcu4KFZOKtM here are a multitude of spells being cast in the BG series. Each has their own feel to them based on the words used, colors, motions, etc. As is, each spell in the game has a very 'samey' feel to them because of that obnoxious effect upon selection. It overrides everything else that's done to make spells feel different.
11. Don't force-lock dialogue to the character that happened to hit the trigger first. There is zero reason why a dumb Fighter would be the one trying to determine the magical runes around a door when the party Wizard is literally just sitting there in the background. Adopting a more party inclusive approach to dialogue would go a looooong way towards helping the dialogue feel more compelling to the entire party, rather than just that individual character.