For the Toddler Gohan body, I would highly recommend the Adverge Children bootleg (the ones that come on red bases like WCF) body. That's what I am using here:
res.cloudinary.com/branjita/image/upload/v1643493749/wemprkmypq526imj2umf.jpg The bootlegs are sold as a set that includes DBGT SS kid Goku, DB Chi Chi, DBZ Bulla/Bra, DBZ Pan, DBZ Goten, DBZ kid Trunks, DB Upa, and a few others. I assume you can still find it from China on ebay, aliexpress, or maybe even wish. Just make sure you buy a set that comes on red bases. The ones that come on black bases are HUGE.
A Marvel Legends Abomination figure could be a good stand in for Bio Broly. I have one, and I still haven't customized him.
I definitely understand Jakks Tien and Vegeta being hard to find and expensive. They are also hard to find and expensive in the US as well. The Giochi Preziosi Tien is definitely smaller, he has very short legs unfortunately. The Real Works Saibamen look good. I have one I could probably sell you if you are interested in buying some figures from me in the future. I might have some other figures you'd want, so it could be worthwhile to buy a lot of them to same money on what will surely be expensive shipping. I don't think I have an extra Tien, but I could be wrong.
The best option for you for Saiyan Saga Krillin, since you don't want to make an elaborate custom figure, is to get the Red Rooster happy meal toy of Krillin from Australia. He scales extremely well with 5 inch figures. You can see him here in my collection:
res.cloudinary.com/branjita/image/upload/v1645990132/ftrgcqgicce73kap0pb5.jpgI'm not sure if I have an extra Jakks Final Form Frieza... I might though... I think I have the body of one, that I was going to put a Chara Puchi head on... and if that's true, I could sell you my Jakks Final Form Frieza with battle damage... but I'd have to look. The battle damaged Piccolo with half shirt is a figure I've never owned, but he's a lot bigger than the other Jakks and Irwin Piccolos. I thought about using one for making into King Piccolo, but I never found one cheap enough in the time limit I set for myself years ago.
You could probably make a good amount of money selling your Demoniacal Fit Guldo and replacing him with a DWC (WCF) or DWC bootleg one. I'm not sure if the bootleg ones are still possible to find, but he was very reasonable quality for a bootleg.
The best Dodoria option is an HQ DX one.
res.cloudinary.com/branjita/image/upload/v1643992595/csrjyywwakygre8at56o.jpg The HSCF Zarbon is an ok option, but I have mine standing on top of a platform so he doesn't look so small. In reality, he's around the same height as Kuwi/Cui next to him.
A Real Works SS Gohan head on a Real Works Vegeta body works ok for Gohan in armor.
res.cloudinary.com/branjita/image/upload/v1644604044/hakt5q8coesppikqby7p.jpg I think I shortened his legs a little bit.
The SHF Chaozu that came with the Cell Saga Tien would definitely be the easiest and best option for Cell Saga Chaozu. Expensive to get though.
Chara Puchi Goten and Trunks heads can be put on Irwin SS Goten and SS Trunks bodies without very much trouble.
res.cloudinary.com/branjita/image/upload/v1645219837/lwiraiikkqbenlurdvxl.jpgI'd recommend getting either the DeAgostini Kibitoshin from Italy or the Kidz Biz (UK) or AB (France) Kibitoshin. They are both much better sizes than the Jakks one. I prefer the DeAgostini.I previously owned the AB one, but the DeAgostini is better looking and a better size.
res.cloudinary.com/branjita/image/upload/v1645219813/d0jo7wtsd0hkclqecswl.jpgI like your custom DBGT Trunks and SS Trunks a lot! Good idea regarding Ultimate Figure Series Piccolo being great for the Super Hero movie version of Piccolo. I hadn't thought of that. If Bandai doesn't make one, I'll need to see if I can find an old UFS for sale.
For DBGT Kid Goku's head, I think you'd be very satisfied with the SS Goku head from that bootleg Adverge set I mentioned earlier. You can see the one I own in this pic.
res.cloudinary.com/branjita/image/upload/v1647624327/guanehxtbmxe3zy3fo48.jpg It's extremely nice looking for a bootleg.
res.cloudinary.com/branjita/image/upload/v1647624390/ee86rtcbaubd15bgwqyu.jpg I don't remember if there's a non-SS version in that set or not. I sold half of that set to someone else shortly after buying it many years ago.