How can I quickly put all the parts together to create release stages?

[LUCINNDA]

I have 225 engines. Since radial symmetry cannot be used, due to the well-known bug, I have to put them one by one. That creates me 225 stages. The work to join the 225 engines in a single sequence is enormous, but it is also that I have to remove the 225 free gaps that are created. Please tell me that yes, there is a way to delete all the empty sequences without having to do it one by one. And it would be great if there was a way to join parts together by grouping them in some way.

[The Aziz]

I want to see the monstrosity you've built to need 225 engines.

Anyway, are you picking every single one from the part selector? I found that, at least in some cases, copying the part with Alt seems to keep them in one stage. But dunno to what point.

[LUCINNDA]

Monstrosity? Is just one of 5 parts of my Star Gater. You will see soon....
Im trying differents ways to group motors while I`m building the vessel. 

[GalileOh-No]

I'm not sure if you want to go with mods, but if so here is an option I've seen on Space Dock:  https://spacedock.info/mod/3354/Lux's OAB Extensions

[Vanamonde]

Why is each engine being placed in its own stage? How are you placing them? 

[LUCINNDA]

 I placed in a motor bay 8 radial motor and one center. One bay per tube and is one tube and 6 around. And I have 4 of them. 
Every time each motor take his own place alone in stage bar. Then I have to join all of them ( a lot) and the free slots, some time persists, and some times auto erase. When persists, the times for load for launch turned long and long... So I have look for a way to erase free stages slots. 
Will be nice if we colud take/select lot of slots and an option to join in one, or group all same type... 
Every thing is easy for normal sice vessels, but if you want to let your imagination to fly and buil something realy big (you will see soon) then all is very very harder.
My PC: I5 12400F - 32Gb DDR4 3200 - NVIDIA GForce RTX 3060Ti 8Gb