I love that you want to give your future birds such a large space to enjoy life in captivity. I personally have grown to hate keeping birds in cages too small for them to fly in, which includes all commercially available cages. Even what's called a "large" (1m x .5m x 1m) ,or "flight" cage is too small for budgies to do more than flap a couple times from end to end, and it makes me sad to see them so confined. This led me to allow 11 of my budgies free fly in my living room from 8am to 8pm daily. They make delightful, if messy, housemates! Some are more human friendly than others but after almost a year living this idyllic life, none of them are afraid of people at all. They're out even when we're not home and never get into any trouble. I have a great bird sitter for when we're out of town.
An aviary 4 meters long, almost a meter deep and 1.5 meters high is large enough to happily house just about any smaller birds you want as long as they're compatible. I don't know if multiple lovebirds are compatible in any size cage, especially in breeding condition. That's a species specific question I can't answer and to some extent an individual bird specific question that nobody can predict. Some birds just don't get along regardless of the size of the cage or aviary.
I do know that a small flock of budgies (12 to 20) would (usually) do great in such an aviary as long as you don't add nest boxes. There are exceptions however. A great example is my mixed sex flock of 11 free flying budgies can't all fly in the same room because I have two young males, Tiki and Baja, that are sexually aggressive toward any females, chasing them around to try to mate without even trying to form a pair bond first. My girls don't like it at all and try to get away from them, getting chased around the room, so I let these two "rapists" free fly in an adjacent enclosed porch to keep the peace. Aside from this behavior, Tiki and Baja are good boys. They're tame and human friendly and I wish I could keep all of them out in the same room but I can't and expect peace and harmony within the flock. You could have this problem with any species of small bird you try to keep in your proposed aviary so I strongly suggest you have a removable divider(s) in the aviary to section it off if necessary.
If you could build an aviary of those proportions that allowed you to place two dividers at one meter intervals you would have a perfect set up to allow not only separate incompatible individuals, but also allow breeding one pair at a time in one 1/3 end section while letting the rest of your small flock enjoy 2/3 of the space.
Heres where this LONG post gets complicated and even LONGER so sign off if its too much, but the big question is how are you going to build such an ideal aviary out of bird safe materials? Zinc coated galvanized metal is toxic to birds if they ingest any small bits of it while chewing on the bars as all hookbills will do to some extent. It's not so much with softbilled birds like finches but not ideal either. PVC coated wire is better but stainless steel or powder coated metal would be best but are difficult to acquire and stainless steel is very expensive.
One option is to purchase four large 32x18x36 powder coated metal cages and use the panels to fabricate the size cage you want by drilling additional holes in the metal frames where needed using a drill bit for metal and using nuts and bolts to attach the panels how you want. I've done this myself to make larger cages out of smaller ones.
The back of the aviary can be a solid piece of thick PVC board they can't chew and damage, will give the aviary the structural integrity it will need so it doesn't collapse, keeps some debris inside, and allows you to use nuts and bolts and L brackets at all corners to hold the structure securely together.
The front can be made from the pre-made cage fronts with doors, bolted together with long bolts to allow spaces between the front panels to insert dividers if necessary, with the premade spaces at the bottom for pull out debris trays. You can use the bottom trays that come with the cages or bottom trays can be made from PVC sheets with one inch deep sides and a front piece with pull knobs.
The top can be easily made from the back cage panels.
The two end sides can be made from side panels of the four cages used sideways and toward the top, with the bottom 1/3 portion of each far end made from solid PVC board bolted to the solid PVC back and bottom for needed strength and structural integrity and using L brackets to give it shape and strength at each corner top and bottom. 10 cm wide strips of PVC board can be used at the top between the front panels, front to back to give it needed strength and shape. The side cage panels can be attached with nuts and bolts directly to the back PVC board, bottom side PVC boards, and the front cage panels. I'd leave the top cage panels just resting on the top for easy removal. Metal L brackets at each corner, top, bottom and sides would help keep the structure square and solid.
The bottom of the aviary can be a solid piece of thick pvc board that's bolted to the back and side lower PVC boards keep all debris inside, again using L brackets at corners to keep the structure square and intact. Melanine board can be subbed for PVC board but I think PVC board is probably better. I hope you can picture this structure I'm describing. It's a PVC board (or melamine board) box with cage panel front, upper sides and top with pull out debris trays.
I apologize to all for this long winded post but this is a topic I've spent a lot of time on myself before OP brought it up on PF and there may be others interested in making their own custom large cage or small aviary. Fabricating one put of bird safe cage panels is a good option that's not too expensive, readily avaiable, can't be destroyed by smaller parrots, and doesn't require master carpentry skills. I'm sure my very simplified instructions are not detailed enough but are just meant to give ideas and guidance. I hope it helps make a nice, safe aviary for OP's future birds!
