I think to negotiate, you'd have to explain to them why paying you to go now is in their interests - especially if you might resign later anyway, with no severance payable.
Purposes of severance
The point of severance is to promote the security of your job and wage, and to cover transition costs if the company tells you to go (potentially at a time when you do not wish to go, and have nothing else lined up).
For the employer, the payability of severance is also another way of expressing the value of retaining existing staff in their financial calculus.
Particularly in software, retaining people who understand a long span of development history, and who have the capability to navigate a large and complicated codebase and understand all the main concepts there, is extremely valuable. A computerised system typically becomes a "dead hand" once all original development staff have departed.
A readiness to react to faults, an ability to control the organisation and influence it rationally to avoid disruption to systems, or an ability to periodically reproduce knowledge and convey "institutional memory" to new staff, may be an extremely valuable resource for the company even if it requires next to no work (in terms of time or subjective effort) from the worker.
The loss of this knowledge capital would be a hidden cost for your employer, but severance helps to make it real for those in the company who know the price of everything, but who have little operational understanding to factor the value of retained knowledge. It effectively makes the hit of your departure a double-whammy for the employer, but it at least means one of the whammies is quantified and foreseen (instead of none of them being quantified and foreseen, which would make it especially likely that the management will behave dysfunctionally and maladminister the corporation in relation to its best interests in terms of staff retention).
There are also other functions which severance performs for the employer, unrelated to the issue at hand, such as deterring premature departures of staff when reorganisations or redundancies are suspected to be afoot.
The reason I rehearse all this is to demonstrate why demanding even a partial severance may be seen as unreasonable.
They are not forcing you to go on their schedule (or at all), so there is no justification for expecting your transition costs to another job to be covered.
And if you leave voluntarily, you are imposing the hidden cost of your departure - a departure which the severance exists to deter the employer from imposing on themselves too lightly.
And the prospect of you losing the protection of your severance, by abandoning the job on your own initiative, is exactly a lock-in the employer wants to wield over you!
You have approached the matter as if the employer really wants you to go, but are frightened of a large severance. On the contrary, the existence of such a large severance in the first place is a sign that they don't usually want people like you to go - and the law is structured in a way that predominantly requires them to induce your consent to stay, and gild your cage (rather than putting you in leg-irons, for example).
On this logic, why would they pay you even a single penny to leave, when the whole logic is to stop you leaving?
There would perhaps be an argument that they were seeking to get rid of you, free of severance cost, if they redeployed you to toilet-cleaning duty.
But it doesn't sound like they are redeploying you to a role that a reasonable worker would find immediately objectionable - it's not a penal duty, or a duty where you'd be hassled for immediate results despite being inexperienced.
If you really want to leave, you can start interviewing, and take your time knowing you're in a solid job in the meantime.
However, your employer's representatives don't necessarily have to share all the above analysis - either because it doesn't apply in your case, or because they have only a dim grasp of the policy issues at stake.
If you really are set on leaving, then my suggestion would be to reiterate that you think your internal options are a poor fit for your skills and experience, and probe their appetite for buying out your job security in this instance.
Avoid, obviously, suggesting that you intend to leave under all circumstances (which would potentially eliminate your bargaining power), and avoid any whiff of extortion (such as threatening to leave after they've sunk costs on near-term training costs, or otherwise assuming an entitlement to severance when resigning).
If there are near-term training costs, they will be acutely aware of this, and acutely aware of the prospect of incurring these then facing your unconditional resignation.
Instead suggest that you really see this as a redundancy situation and that you're unclear how happy you will be with any of the offered internal redeployments. The impression you want to give is that your future performance might be sullen and perfunctory, lacking enthusiasm because of misfitting, but short of ill-will or downright non-performance.
By casting the obstacle to your leaving as being your own job security, and by avoiding any suggestion that you will fail to perform if they decide they'd prefer to retain you, you're talking turkey with them and offering real alternatives for them to consider.
Also, crucially, the choice must be cast as between them either retaining you, or exploring the possibility of them severing you for less than the usual amount. It mustn't be cast as you offering to resign for more than the usual amount of zero!