By approximate weight/volume, a simple 3x2x2 chest (coincidentally the size of a Leomund's secret chest) is capable of holding at least half a million coins (although it will weigh about six tons), with some shake space to spare. The best solution, at high levels, is to encourage your players to settle somewhere and set up a homestead or a keep where they can plunk a chest down. Nothing beats having a proper coffer of gold at your disposal, other than having a proper vault to protect it. The drawback is you have to leave it behind.
A bag of holding can only carry just so much, after all, and I wouldn't advise handing a party more than one. Encourage the use of wagons and carts for short-distance transportation of a dragon hoard. You can SAY you slew a dragon, but nothing proves it faster than a 16th century U-Haul rolled right through town packed with dragon swag (and its SKULL!).
Some of the ways a DM can assist with long range wealth transportation is by Trade Guild Writs or Trade Bars. These are usually backed by pretty strict laws against fraud and forgery by both the guilds or players alike. As long as you're in the same kingdom under the same laws, there shouldn't be a problem unless you're trying to pass off a forgery (which requires more information than just scribbling some words on some paper). These will generally be of no use in small villages or in the wilderness, but any trade-hub town or city should have guild offices.
Trade Writs are a service offered to Guild Artisans/Merchants in good standing. This is a simple voucher for a certain amount of value credit, which can be exchanged for gold within another Guilds' means, but largely for goods & services All these records are maintained in guild ledgers. If the players have a Keep and a guildsman on hire (or in the party), these can easily be sent home by messenger and the guild closest to the keep becomes a safe reserve of payable wages, materials, labor, and accurate accounting of all transactions. The two cities/towns can work out the debt between them in the background just fine without disrupting economies.
A 'Trade Bar" is a serial-marked bar of brass or a wood token (weighing no more than 1lb) that can be exchanged for a value of 100, 500, or 1000 gold. Players can exchange trade bars for gold once the local guild has received an updated ledger marking its serial number as having been issued, usually taking no longer than 30 days. These can be an alternative to the 'sack of gems' style of wealth compression, since truly valuable gems are both rare and in high demand by people of means. This is generally a safe means of transport, as the bars can have a variety of simple security features to ferret out all but the most elaborate forgeries. These are more 'liquid' than writs, assuming a merchant is willing to accept it, and they usually will prefer a simple bar of fixed value over a pile of gems of suspect value.
Innkeepers often have a strongbox that can be rented for a small fee of a few silver and can look after small amounts of wealth or small items during your stay. It's generally considered safe, as long as you're not a wealthy-looking individual paying for a room in a low-rent area to cheap out on expenses. The better the accomodations, the more secure you can safely assume it would be.