I have played the game and finished the main game on version 1.0.5, I also had played Didnapper 1.8.1 5 years ago. Compared to to the first game, Didnapper 2 feels more like an RPG with exploration rather than being mission/level-based though still using the bondage theme and the battle mechanic from its predecessor. A notable difference between this game and its predecessor is that cutscenes are very frequent in Didnapper 2 though RPGs are usually story driven and it's not uncommon for RPGs to have a lot of cutscenes, take the Final Fantasy games for example. Another difference is that Seles/party gets tied up more often than Suki did in Didnapper. The escape system also gets more use in this game which I felt was underused in Didnapper unless you lost battles.
My review
Below is my review and thoughts of the game.
I do like how the game has an entirely original soundtrack with each town and field location having unique tracks compared to Didnapper using some tracks from RPG Maker XP. I also like the voice acting for the major characters although they're not fully voiced.
I also do like the art for the enemies and some CG scenes as well as the bust graphics being animated. The maps are also neat with the FSM tilesets and parallaxing being used although the Desert areas seem to use the default MV tilesets. I like the taller characters which are close to the RPG Maker XP characters than the chibi characters made by MV/MZ's character generator. Much like Didnapper using the battle backgrounds from XP, D2 also uses the backgrounds from MV maybe with some exceptions like the ship interior.
The gameplay is also improved with the player being able to save anywhere and being able to dash. The battle system uses ATB (which RPG Maker MZ would later include) along with the RPG Maker MV mechanics and battle mechanics from Didnapper like tying up female enemies to win and the party members not being able to be knocked out though with changes like the Tied state operating on points that is affected by Struggle and Rescue. The Escape System is also more complex with different restraint types and tie types which is otherwise similar to the Didnapper 1.7/1.8 escape system. Lastly, Seles can ambush male enemies which Suki couldn't do in Didnapper.
Difficulty
As for the difficulty, it can seem harder than Didnapper (especially 1.8) due to the ATB battle system which works similar to Final Fantasy X where not everyone gets an equal turn, characters no longer being healed after battle, enemies being smarter with Subdue (no longer wasting it on a healed character), and Rescue longer removes Tied instantly. However, you get a lot more Gold, get a character who has healing spells later in Chapter 3, can ambush male enemies which you couldn't in Didnapper, and some areas also have beds to restore HP/CP. Also, the player can easily escape guards due to Seles being able to dash.
I believed enemies used to be focused on using Rescue on the Demo 2 (2019) and possibly the old Patreon versions which made tying up their allies ineffective. In the final game, they don't seem to use it often.
I haven't found anything in the story that's too difficult or easy although things starts getting harder as of Chapter 6. The bosses can be challenging especially in the later half of the game, but I haven't found them too difficult or easy (except the end of Chapter 13) which is a contrast to Didnapper which had difficult bosses like Hyranda in Chapter 3 and the pirate ambush in Chapter 6. Even the final boss is improved to be challenging with having damage resistances, a lot of health (2nd part having 7664 HP on hard), and powerful attacks. It's a vast improvement over the final boss in the first game who didn't seem hard when I played it.
Issues
One complaint I have is that you can't escape non-boss/scripted battles which gets worse later in the game where enemies have a lot of health especially on Hard which can drag on the battle considering that most enemies don't have elemental weaknesses and your attacks don't do high amount of damage even with 4 characters, not helping that Eileen initially lacks damaging spells to match Seles' damage output. On the other hand, you can choose not to fight them (or ambush them instead) though it won't help in areas like the Miu Miu Forest due to the trees, Stromm Marshes with the hidden bandits, and pretty much any location where you can't easily avoid the enemies without triggering a battle.
I've found debuff skills to be pretty much useless in the later game especially against bosses since the enemies' ATB tends to fill quickly before the player can get a turn. It limits usefulness of skills like Kaie's Darken which lasts for 2 turns, but the enemies will usually get those 2 turns before you can cast it. However, status effects are still effective.
Aside from Eileen's Soothing Mist (10% HP) and Liliana's Ration (with healing item), there's no way to heal all characters. It becomes a problem in the later chapters as the multiple enemies can lower your HP with multiple strikes or have skills that hit the party which makes healing to prevent being tied difficult. It doesn't help that Liliana only gets used in 2 chapters and isn't usable until Chapter 13 (who then gets removed until late Chapter 14). Even then, the Ration can be impractial to set up since she needs 30 TP and and two turns which will likely give enemies enough time to weaken and/or tie up the characters.
Despite the issues mentioned above, it's a very good game that improves on Didnapper's gameplay mechanincs especially with Didnapper 2 being built on RPG Maker MV that's more flexible than what RPG Maker XP can achieve. The macOS and Linux versions are also a nice addition although RPG Maker MV is no longer supported on macOS Big Sur 11.0 and later (not sure about games deployed for Mac).